So I have a character that is Level 10 Overall but is Wizard 6 and Rogue 4. He started out as a Wizard at 1st level. For some reason he has a 4th level Spell Slot even though he cannot learn 4th level spells until Wizard 7. Why does he have a 4th level slot? 

I assume you're an Arcane Trickster Rogue? If so - that means you have two classes with the Spellcasting feature and are therefore using the Multiclassing Spellcasting rules which you really should read.

Edit: Using the PHB link above instead of the Basic Rules link because the basic rules do not include Arcane Tricksters.

Quote from Pugsamillion >>

So I have a character that is Level 10 Overall but is Wizard 6 and Rogue 4. He started out as a Wizard at 1st level. For some reason he has a 4th level Spell Slot even though he cannot learn 4th level spells until Wizard 7. Why does he have a 4th level slot? 

Spell slots are determined for classes with the Spellcasting feature (this does not include Warlocks, Four Elements Monks, etc) as follows:

1. Take all of your levels in full caster classes.
2. Take half of your levels in half caster classes, individually.
   1. If you only have 1 caster class, round this up.
   2. If you have at least 2 caster classes, round Ranger and Paladin levels down.
3. Take one third of your levels in one third caster classes, individually.
   1. If you only have 1 caster class, round this up.
   2. If you have at least 2 caster classes, round this down.

So wizard 6/arcane trickster 4 is 6 + floor(4/3) = 7, so you have the slots of a 7th level caster.  If you're any other Rogue subclass, you should have the slots of a 6th level caster.

Quote from Emmber >>

I assume you're an Arcane Trickster Rogue? If so - that means you have two classes with the Spellcasting feature and are therefore using the Multiclassing Spellcasting rules which you really should read.

Edit: Using the PHB link above instead of the Basic Rules link because the basic rules do not include Arcane Tricksters.

 Don't know if this was intended but the way it's phrased sounds rather rude. 

No rudeness intended. Just confusion.

I have autism and rules are incredibly important to me - so it always amazes and baffles me by the number of people who multiclass but don't read the multiclassing rules first. That's just not a concept I can wrap my head around.