Wizard mechanics that are RAW, but not possible to use

Cleric 4/Wizard 1 (in 5e not sure about 5.5e)

Multiclass rules:  [my comments in brackets]

Once you have the Spellcasting feature from more than one class, use the rules below.

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, and half your levels (rounded down) in the paladin and ranger classes. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.

[Char posesses 3rd lvl spell slots, and must use the Multiclass Spellcaster table for spell slots]

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

[A Wizard does not "know" spells, only cantrips, and a Wizard prepares spells that have been scribed to his/her spellbook]

Wizard rules for preparing spells:

You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

[Wizard 1 can prepare any lvl of spell as long as you have the spell slots, in this case 3rd lvl]

When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.

[3rd lvl spells can be prepared, so 3rd lvl spells can be copied into the spellbook at Cleric 4/Wizard 1]

Each time you gain a wizard level, you can add two wizard spells of your choice to your spellbook for free. Each of these spells must be of a level for which you have spell slots, as shown on the Wizard table.

[Multiclass table must be used for spell slots, not Wizard, meaning two 3rd lvl spells can be added to the spellbook for free at Cleric 4/Wizard 1]

[Currently, this is not possible to accomplish using dnd beyond, probably because the fact that Wizard casting being reliant on available spell slots, not limited by the Wizard's lvl, was overlooked at some point.  RAW it should be possible to do.  In the mean time, let me know if there is a way to forcibly add higher lvl spells to a Wizard's spellbook using dnd beyond.]

[Apologies if this should have been in game mechanics or the Wizard sections.]

[Cursory glance at 5.5e seems like it should also be possible.]

Seems like Cleric is in the same boat, Wizard 4/Cleric 1 should be able to prepare and cast 3rd lvl Cleric spells.

You prepare the list of cleric spells that are available for you to cast, choosing from the cleric spell list. When you do so, choose a number of cleric spells equal to your Wisdom modifier + your cleric level (minimum of one spell). The spells must be of a level for which you have spell slots.

Upon further inspection, the only Spellcaster classes that are unaffected may be Bard and Sorcerer, due to "knowing" their spells.

This is from the multiclassing section of the 2014 PHB regarding spellcasting. The 2024 rules are basically the same if with slightly different wording.

Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.

You have to determine what Wizard spells you can prepare as if you were a single class character. So if you are a Wizard 4/Cleric 16, you have to determine which Wizard spells you can prepare (and thus copy) as if you were just a 4th level Wizard. The character builder reflects this.

Sequilonis is correct. The multiclassing rules override any reading of the rules from single classes that would contradict them. 

Notably, BG3 allowed wizards to do this. Bug or intended on their part, I can't speak to, but it's well known that they didn't follow the 5e rules exactly.

My understanding was that the lvl 4 cleric can prep those level 2 spells and the 1 Wizard only has level 1. You have access to level 3 SLOTS by your character level but you are Not a level 5 wizard, you should not get Fireball.

You have to determine what Wizard spells you can prepare as if you were a single class character. 

I did this:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. (A Wizard does not "know" spells, only cantrips, and a Wizard prepares spells that have been scribed to his/her spellbook)

The following is the instructions in the Wizard class for preparing spells:

You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

(Wizard 1 can prepare any lvl of spell as long as you have the spell slots, in the Cleric 4/Wizard 1 case, 3rd lvl)

Please re-read the OP, as all rules were followed correctly, it is just that there is a loophole in the rules surrounding casting being dictated by spell slots availability, in the rules for preparing spells, which you are instructed to use the instructions in each class for.

 That is not RAW tho...

You calculate spell slots as a Multiclass Spellcaster.

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

The Wizard single class preparation instructions say the spells must be of a level for which you have spell slots.  Not Wizard spell slots, just spell slots.

The rules for preparing spells tell you to rely on spell slots, which are calculated as a Multiclass Spellcaster.

Quote from wagnarokkr >>

When you determine how many and which spells you can prepare for each class, you pretend like you don't have any levels in any other class.

Where is the RAW that backs that up?

RAW:  You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

Know and prepare only, otherwise use Multiclass Spellcaster rules.

RAW Wizard rules for preparing spells:  You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

What rules are used for having spell slots?  It isn't knowing or preparing, so Multiclass Spellcaster rules are used.

RAW:  You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, and half your levels (rounded down) in the paladin and ranger classes. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.

Quote from Tyran_Scorpi >>

Quote from wagnarokkr >>

When you determine how many and which spells you can prepare for each class, you pretend like you don't have any levels in any other class.

 

Where is the RAW that backs that up?

RAW:  You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

Know and prepare only, otherwise use Multiclass Spellcaster rules.

RAW Wizard rules for preparing spells:  You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

 From the wizard spellcasting feature:

The book contains the level 1+ spells you know

Adding spells to the book is determining what spells you know.

RAW:  You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

You determine the spells you know as if you were a single-classed member of that class.

If you were a single-classed first-level Wizard, you would have only first-level spell slots. You cannot know spells that are not first level. While, you, the character have higher-level slots, a first-level wizard does not, and you are determining your spells as if you were one.

Also, just in case you were uncertain about the interpretation, there is an example in the multiclassed spellcasting rules immediately afterward.

Quote from jl8e >>

Quote from Tyran_Scorpi >>

Quote from wagnarokkr >>

When you determine how many and which spells you can prepare for each class, you pretend like you don't have any levels in any other class.

 

Where is the RAW that backs that up?

RAW:  You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

Know and prepare only, otherwise use Multiclass Spellcaster rules.

RAW Wizard rules for preparing spells:  You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

 From the wizard spellcasting feature:

The book contains the level 1+ spells you know

Adding spells to the book is determining what spells you know.

RAW:  You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. 

You determine the spells you know as if you were a single-classed member of that class.

If you were a single-classed first-level Wizard, you would have only first-level spell slots. You cannot know spells that are not first level. While, you, the character have higher-level slots, a first-level wizard does not, and you are determining your spells as if you were one.

"Adding spells to the book is determining what spells you know."

RAW: When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.

You can add any spell that you could prepare to the spellbook.

RAW Wizard rules for preparing spells: You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook equal to your Intelligence modifier + your wizard level (minimum of one spell). The spells must be of a level for which you have spell slots.

Rules that define what can be added to the spellbook rely on the rules for preparing spells, which rely on spell slots, which as previously pointed out use the Multiclass Spellcaster rules.  So my position is correct RAW.

Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

Quote from wagnarokkr >>

Quote from Tyran_Scorpi >>

So my position is correct RAW.

Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

Is it your belief that you're the first person to have "discovered" this "loophole" in the last twelve years?

 No clue, but the fact that it isn't possible using dnd beyond suggests that it hadn't been spotted by anyone that could have made changes to how the Wizard class functions on the site.

Quote from wagnarokkr >>

Quote from Tyran_Scorpi >>

Quote from wagnarokkr >>

Quote from Tyran_Scorpi >>

So my position is correct RAW.

Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

Is it your belief that you're the first person to have "discovered" this "loophole" in the last twelve years?

 No clue, but the fact that it isn't possible using dnd beyond suggests that it hadn't been spotted by someone.

It isn't possible using D&D Beyond because the people implementing D&D Beyond understood how the rules work in this area.

jl83 already mentioned this, but the rules themselves do include a specific example that demonstrates that your interpretation is not correct. It says:

Spells Prepared. You determine what spells you can prepare for each class individually, as if you were a single-classed member of that class. If you are a level 4 Ranger / level 3 Sorcerer, for example, you can prepare five level 1 Ranger spells, and you can prepare six Sorcerer spells of level 1 or 2 (as well as four Sorcerer cantrips).

In this example, if your interpretation were correct, the multiclass character would be able to prepare level 2 and 3 Ranger and Sorcerer spells, but the example says they cannot. This makes the intent of how this is meant to work quite clear. 

 I am not arguing RAI.  The example does not follow RAW.

Edit:

RAW:  The Spells Known column of the Ranger table shows when you learn more ranger spells of your choice. Each of these spells must be of a level for which you have spell slots.

Ranger also relying on spell slots to determine spells that can be known.

Quote from Tyran_Scorpi >>

 Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

 I would also refer you to the first chapter of the DMG

Rules Rely on Good-Faith Interpretation. The rules assume that everyone reading and interpreting the rules has the interests of the group’s fun at heart and is reading the rules in that light.

The idea that a subtle read of the wizard's spellcasting feature lets you get around the clearly-stated and exampled rules for multi-classed spellcasters is in no way a good-faith interpretation.

You've said your piece. You've had it explained in detail why you're wrong. I, at least, am not interested in having this discussion yet again.

Quote from jl8e >>

Quote from Tyran_Scorpi >>

 Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

 I would also refer you to the first chapter of the DMG

Rules Rely on Good-Faith Interpretation. The rules assume that everyone reading and interpreting the rules has the interests of the group’s fun at heart and is reading the rules in that light.

The idea that a subtle read of the wizard's spellcasting feature lets you get around the clearly-stated and exampled rules for multi-classed spellcasters is in no way a good-faith interpretation.

You've said your piece. You've had it explained in detail why you're wrong. I, at least, am not interested in having this discussion yet again.

 

 That is fine, you do you.

That said, I am not wrong RAW, and would like the option to use the rules as written at my table using dnd beyond.  Which is not currently possible.  Hence the post.

If no one wants to address the issue I suppose it is back to good old pen and paper.

 Edit:  for the record, at the table I am playing at, RAW is LAW, and RAI is an inconvenience to be avoided.  Don't gatekeep our fun.

Quote from Tyran_Scorpi >>

Quote from jl8e >>

Quote from Tyran_Scorpi >>

 Yes it goes against the generally accepted interpretation, but that happens when a loophole is discovered.

 I would also refer you to the first chapter of the DMG

Rules Rely on Good-Faith Interpretation. The rules assume that everyone reading and interpreting the rules has the interests of the group’s fun at heart and is reading the rules in that light.

The idea that a subtle read of the wizard's spellcasting feature lets you get around the clearly-stated and exampled rules for multi-classed spellcasters is in no way a good-faith interpretation.

You've said your piece. You've had it explained in detail why you're wrong. I, at least, am not interested in having this discussion yet again.

 

 That is fine, you do you.

That said, I am not wrong RAW, and would like the option to use the rules as written at my table using dnd beyond.  Which is not currently possible.  Hence the post.

If no one wants to address the issue I suppose it is back to good old pen and paper.

 Edit:  for the record, at the table I am playing at, RAW is LAW, and RAI is an inconvenience to be avoided.  Don't gatekeep our fun.

 Your key distinction seems to be the wording around "slots", that the Wizard prepares spells for the slots they have irrespective of the origin of those slots.

However, if you prepared spells as if you were a single classed wizard you would have no other slots available for which to prepare spells.  Therefore the multi-class spell slots do not matter.  

So, RAW you are incorrect and no amount of "loophole" calling will change that.  BUT as far as gatekeeping goes, no one is or can possibly do that.  Your table is your table and you could use THAC0 or any other rules you like.  DDB is under no obligation to cater to your interpretation of a rule, and frankly cannot be expected to accomodate every possible permutation and adjustment of the rules.  It is a tool not the game itself, which is completely open to whatever shenanigans that a table wants to have.

However you play, enjoy your game!