If you are just quad boxing, try grabbing four of the same class to start. Take Shaman for instance. You don't necessarily have to level them to 80, but it makes it much more of a good base starter group to see what you can handle. If you can handle them with ease. Switch it up.
What you choose to level is entirely based on what you want to do in the end.
If you are going to do PvE, non-raid, I suggest 1 Tank(Paly or DK), 1 Healer, and 2 DPS. This is because it is easier to pug a shitty DPS than otherwise, and this way you know you have tank/heals covered.
If you are going to do PvE, raid, I suggest all casters, Sham, Mage, Boomkin. They are mostly all macro/priority based, and can be managed quite easily.
If you are going to do PvP, Shaman seem to be the thing. They can CH, do massive burst, and are quite easily controllable, and have good anti-melee.