how does this work, 

the reason i ask is, i have a character (npc) that started as a first level rogue, but immediately multiclassed at level 2 to cleric, which they will remain as to 20.
so does this character get his first ability score increase when he becomes lvl 4 or lvl 5, 
as when he becomes a 4th lvl cleric he's going to be 5th level, 
this would also mean taking it to an extreme that if you had a character with 5 classes  and it's by class level not character level they could in theory be lvl 15 without getting one ability score increase, which to me seem ridiculous

Your ASIs are based on class progression, not character progression. So when you get to level 5 if you are Rogue 1/Cleric 4 you will receive your first ASI.

That is one of the trade-offs of multi-classing, so it's always important to plan carefully. Your spells are also affected, so that's important to note as well.

I have one player in my group who is a Sorcerer/Cleric who is debating on the trade-offs of upping their Cleric level to 4 vs losing out on learning a 9th level spell at Sorc level 17. It's an interesting balance to examine-- And in the end you make whatever decision seems most fun to you, and take whatever balances/trade-offs that means.

i suspected that may be the case, oh well looks like the homebrew kits going to be coming out then, though it's not a difficult one everyone gets one at 4, 8 12,16 and 19, i'll transfer that to being a consequence of development of the person,  then keep the extras that some classes get as a class feature, it's a homebrew campaign anyway, so not going to affect the wider world 

Losing ASI is one of the reasons why I'm not a fan of multi-classing.  Sticking with a single class has benefits and ASI is one of them.  Higher level spell slots is another one for spell casters.

I agree with Stormknight... If you do this, you may want to give characters who *don't* multiclass some other benefit, as otherwise multiclass characters will get a powerspike.

And don't forget about classes that get more ASIs in their progression, like the Fighter or the Rogue.

Yep, absolutely @Onyx.

It's important to remember, OP, that 5e was rigorously designed and tested. There may be some minute edge-case rules that need a little helping hand or DM fiat, but the core mechanics are pretty well balanced. Multiclassing is one of these core mechanics, if you change the balance in one way you either need to right the scales the other way, or you could create a severe power gap between members of your party, which has an unfortunate side effect of resentment. "Wait why don't I get something extra at ASI? Can I get another one? Just because I didn't multiclass I miss out?" that kind of thing.

They're already covered, they only come with that class level, it's only the ones that everyone gets at 4,8,12,16 and 19, that come at personal level, so to get the rogue 10 extra one you would indeed have to actually be a rogue 10, that is a class feature.

But what are you going to do for the people who don't multiclass at levels 4, 8, 12, 16 and 19 to re-balance the game? 

Give them their stat increase they will after all qualify for it by being that level

Please be aware that doing this may create some really odd power spikes for multiclass characters, as the ASI is considered to be a class feature.

If you remove the ASI as a class feature and don't replace it, then it grants more power to multiclass characters.

It's important to remember, OP, that 5e was rigorously designed and tested. There may be some minute edge-case rules that need a little helping hand or DM fiat, but the core mechanics are pretty well balanced. Multiclassing is one of these core mechanics, if you change the balance in one way you either need to right the scales the other way, or you could create a severe power gap between members of your party, which has an unfortunate side effect of resentment. "Wait why don't I get something extra at ASI? Can I get another one? Just because I didn't multiclass I miss out?" that kind of thing.

Where's the power gap, at lvl 4 they both get one stat increase at lvl 8 they both get one stat increase at lvl 12 they both get n stat increase T lvl 16 they both get one stat increase at lvl 19 they both get one stat increase, they're exactly the same, doesn't mean he becomes a lvl 4 cleric with all the class abilities of a lvl 4 cleric, he's still a cleric 3 with the stat increase of every 4th  level adventuring class person, just happens to have 1 of his other development levels as a rogue, the story still revolves around the paladin. Not his sidekick

Because if you multiclass, you're usually gaining access to extremely helpful features. Doing a quick dip into fighter can get you action surge or 2 into Druid for wildshape, quick dip into Monk or Barbarian gives you unarmored defense -- plus rage and unarmed attacks for each. Dip into Rogue for sneak attack. Dip into any caster class for cantrips, which grow based on character, and not class level. Dip into Cleric and you get a whole wealth of features from the Divine Domain, which in almost all cases are fantastic. I have a sorc with a 1 level dip in tempest cleric who is a-mazing. This is what you gain in return for giving up 1 or 2 levels of progression in your other class. That's the trade off that creates the balance of multiclassing. If you can do the dip without setting yourself back, it makes not multiclassing decidedly weaker.

The fact that you want this so bad is a good hint that it's a lot more unbalancing than you're giving it credit for.

It also makes leveling up in a single class less fun, because whenever you hit 4/8/12/16/19 in a class now, you're going to get.... nothing special, since the classes already skip handing out features at ASI levels. Be prepared for everyone to be a charisma-based hexblade/moon druid/fighter/rogue/monk/barbarian/paladin/sorceror, because hey why not

Well done for coming up with a huge number of meta reasons that are never going to happen in this game

Everyone will be, a paladin, till the person playing it decides they've had enough, and it's sidekick which has 1 level in rogue, I suspect the worlds not going to burn down as a consequence of that character getting its stat boosts on the same level numbers as the paladin

You might be shocked how front loaded the useful abilities for each class are.  That is to say that you are making a mistake if you do not give something back to your straight class characters if you wish to follow your current line of thinking.

We are not telling to avoid your line of thinking altogether but you are being ignorant of how the balancing for class abilities and character progression has been laid out.  At the very least if you aren't going to give your straight class characters some minor perk to make up for it you better give them more potent magic items than the multiclassers, or an extra feat, or even the option for getting some kind of monstrous transformation (lycanthropy, undead).

These people are trying to help you have the best possible DnD game you can have.  They are not out to get you, they are out to assist you.  Take in some of their knowledge and please consider what they have said.

Is there some method of closing threads this one died at post 2, it's beyond death and either needs closing or deleting, 

Actually, I disagree with everyone saying this won't work. It will work just fine, and be balanced just fine. As long as the *only* thing you get at character level 4 is the ASI. So if you were Cleric 2/ Fighter 1, then on your next level you would only get the ASI, no matter which class you leveled up in. All the other class features for that class then just shift up one level to access them. That might need to include new spell levels, but balancing this wouldn't be difficult.