If we're to think about 4 of the same class...
4 Wizards would be able to make a balanced team: Abjurer as your Tank, Diviner as your Support and a couple of Evokers to blow everything up (Or 1 Evoker and 1 Necromancer if you wanted to have a summoner).
4 Druids can also make a balanced team: 1 Moon as a tank in Bear, 1 Moon as melee damage in Saber-tooth, 1 Land for spell damage, 1 Spore for support.
4 Cleric would be interesting. Early Act 1 would be kind of tough, but Act 2 would be a breeze with quad Spirit Guardian obliterating everything.
4 Assassins would be interesting. Try and nuke down as much as you can in the first round!
