For 2v2, I guess you can learn a lot just by looking some YouTube videos and practicing...
For 3v3, there are existing articles out there:
- Basic Tips
- Early-Game: Defensive Builds
- Mid-Game: Protoss
- Mid-Game: Terran
- Mid-Game: Zerg
- End-Game: Options
I guess that once you're good at the above the 4v4 doesn't differ much, I wouldn't suggest playing it either as you will need to find more people and will have to wait longer. It isn't competitive either, as Tzenes has pointed out earlier in one of his answers. The maps aren't made for it which can make the choice of races unbalanced...
unit synergy of different races
I would suggest every race to create his own composition of units, why?
If you create only one type of units instead and your friend the others,
you will depend much more heavy on one resource. (Example: Marines would be minerals only)When controlling your army, if you are alone it's much more easier to command your different types of units so they don't accidently die. Then you avoid sending in vikings over a group of enemy marines... If you were separated, you would have to share such thoughts with your friends and then his vikings might arrive to early or too late.
When talking about different races, you can ask and plan what units to create to accompany your friends' army. If he's planning an air assault, you could create some ground units just to distract the opponent away from the air as you both attack...