My general rule is 2 melee, 2 ranged, with one of each class.
That could be:
Setup 1- Warrior Tank, DPS Warrior, Ranged Rogue, Support Mage
This is a fairly easy way to play, considering you have two characters on the frontlines who can soak up damage. The ranged rogue is relegated to hanging in the back and picking off enemies and/or crowd control and the support mage just pretty much ensures everyone's defenses are up, bodies are hastened and they're still alive.
Setup 2- Warrior Tank, Melee Rogue, CC Mage, Support Mage
I love this setup. The tank draws aggro and soaks up damage, preferably from crowds and the hard hitting bosses/elites. The CC mage keeps people disabled and controlled while the melee rogue (ideally the PC, but I'm thinking Cole will be very good at this if not better in DAI) picks off the mooks one by one or hits the elite/boss with constant backstabs. The support mage, once again, just makes sure everyone's alive and occasionally fires off a disabling or attack spell just to be useful.
Setup 3- Warrior Tank, Melee Rogue, Ranged Rogue, CC/Support Mage
This one's a bit tougher because you have less spellpower (than Setup 2) and less tankiness/more squishiness (than Setup 1). The ranged rogue can take on the role of crowd control OR ranged dps, but ideally the mage focuses on crowd control when he/she is not on support. The rogues pick off enemies while the warrior tank does what he/she does best, taking damage and drawing attention.
As long as there is 1 designated Warrior Tank and 1 designated Support Mage I'm good. With no healers in DAI, this pretty much means I'll be hybridizing my support mage to defense (barriers) and crowd control while the other mage (if I go with Setup 2) will be pure dps/AoE.