Welcome!!

My setup is similar prot pally, priest, mage, and lock (working on leveling a 2nd mage to make 5). Its a good setup for dungeons and instances. I am also finding its not to bad for pvp. Were I in your shoes I would consider 3 mages instead of 3 locks. What I have found is its all about the quick kill. Mages just have better burst damage than locks(imho). 3x firebolts or 3x arcane explosions just rocks.