I was going to roll 4x lock 1x priest, but I decided to toss a lock for a paladin. Here's my reasoning:

For PvE: Prot Pally, Holy Priest
For PvP: Holy Pally, Shadow Priest

That way you don't lose a slot to a tank.

PvE: 4x lock, 1x priest cannot do heroics (afaik) or they'll be very very hard
PvE: 1x paladin, 3x lock, 1 priest can do almost any heroic (afaik)

PvP: 4x lock, 1x priest will melt faces, do doubts here
PvP: 1x paladin, 3x lock, 1x priest will melt faces slightly less, but melt them none the less


In exchange for 1 warlock you gain:
KINGS +10% stats (hp most importantly) which means your 11k hps just became 12.1k hps, or +>5000 hp across the party.
AURAS non-pushback spell casting w/ conc, and other auras are useful too
Oh-crap button -- bubble
Plate-wearer to take brunt of the focus fire
If you are horde-- AOE silence
A ranged stun (HoJ)