Between '5 locks vs 1 priest 4 locks' I would do 4Warlocks+Priest.

You should also consider 3Warlocks+Priest+Paladin as it gives you a trinity option yet still does decent AoE as the 3 locks don't lose any AoE DPS due to push-back because the Paladin's Concentration Aura makes them 100% immune to push-back.