I have 4x shadow priest myself at level 70, in pvp its fun.. but really gear dependant as you hit the ground kinda fast against good geared people

4x Mind blast + SWD combo is a LOT of dmg

downside nowadays is the vampiric embrace, and the mana dot being nerfed to hell.. no more unlimited mana.

So further then that.. id just go for 4x fire mages, would be the same i suppose
Just costs too much mana to go out of shadow form to heal and back, VE doesnt really heal enough anymore to be good.