You'll probably want to bring a 5th with that is fine with each of you dual-boxing.

To make it feasible in a lot of instances, make sure one of you has direct control over the tank and another over the healer.

Ranged dps is obviously what you should aimed for, be in Shaman, Mage, Warlock, Spriest, anything.

Have your DPS auto follow whoever is in control of the healer, tank needs to have a lot of focus for positioning mobs, etc. That when when you are good and ready you can start unloading/ccing with your 2nd box. Being the tank and the healer is just too much for most instances, especially with AOE damage, much better to have someone that can focus 100% on the heals when needed.

Good luck.