I'm still not sure what the devs were pushing for with our 4pc. Sure, the 4pc will be an awesome way to try out 5.0 mechanics, but mind spike is still far too weak to spam single target over dots. However, as a multitarget cooldown... (ei Dots up on the main boss, shadowfiend+mindpspike spam on a secondary target) the set bonus might be very effective.
Personally, I still feel the shadowfiend portion or the bonus is rather weak... and expect to be rather disappointed by it. I've wanted to do more theorycrafting on this set bonus, but I've been so bogged down between RL issues, and trying to get this 4.3 BiS list ready to go.