A very basic choice...

a) Tank + 3/4x DPS that can heal, possibly with another class.
b) Tank + 3x DPS + 1x Dedicated healer.

Both approaches have merit, strengths and weaknesses.

Another option, I'm seeing people having success with:
Tank + 2x Healers + 2x DPS.
Especially if the 2x Healers can add some amount of DPS to almost equal a 3rd DPS.
Gunsun has a thread on this composition, with others trying several variants.
2x Holy Pallies, each doing Beacon on 2x Elemental Shammies and then healing 1x Tank, is an example; each heal on the tank, hits the entire group with 1x Heal per other toon and 2x Heal on the tank.