If you're going with 4x warlocks, I'd probably have one voidwalker as the tank.
The Felguard is the highest dps pet, with taunts off they shouldn't take threat from the VW.
Even if they do, 50% of your damage heals the pet, and VW should have most of the threat.
The Felhound grants an Intellect buff or the Imp grants Stamina. Not sure if either is stronger then another Felguard.

I'd probably go with 3x Felguard, 1x Blueberry (tank). Demo spec, as its the easiest to box.

As far as heals, go with...

/assist Master Warlock
/cast [help] Regrowth; [help, target=targettarget] Regrowth; [target=VW's Name] Regrowth
Substitute in Rejuvenation, Healing Touch, Lifebloom etc.

The heal is directed towards whoever the mob is attacking.
You can also force the heal on whatever you want, by targeting it on the Master and pressing the desired heal.