Agreed with Aboron. VE heals the priest for 15% of their damage and everyone else for 3% of their damage. I have a shadow priest(with 1800 sp and crappy rotation) in my group and I see +40 to +50 hp ticks from VE. I just started boxing instances with a few shamans that have 1140 sp and get +100 to +130 ticks from healing totem. Neither one of those are good for heroics, however I can forget about healing in regular level 80 instances trash with the 4 shamans. I don't think the 4 spriests will keep me alive on 4 pulls.

I would suggest setting up one priest as disc if you're going with 4 priests. You could checkout my disc priest. I have a /castrandom [no channeling] Power Word: Shield, Prayer of Mending, Penance macro bound to my dps and tank spam key and I heal just fine. 80% overhealing and I don't run out of mana on trash or boss fights, I do use shadowfiend when i hit 50% on bosses though. Anytime anyone other than the tank takes damage, holy nova or prayer of healing. I don't bother with single target healing on my dps or healer anymore.