I am personally going through the same thing too, on deciding the optimal team.

I am working on a 4 player setup:

Right now I have:
Monk for tanking
Templar for Healing
Illusionist for Mana/Dps

Haven't decided what I want to do yet, but thinking a plate tank would be a lot better.
Plate tank
Templar
And not sure about the last 2.

Would a bard be good enough mana regen, or even how much mana regen is needed for a 4 player group?

Seems like we both have the problem with the last 2. I am more biased for casters, and agree 1 of the slots needs to be a good DPS slot.