I played a shadow priest solo forever, like years, and i was shadow most of the time. the only instance i could effectively heal the whole group while in shadow form was Black Morass, we carried a holy pally that would spot heal but he pretty much didn't have to do anything the whole time, the only problem with the spriest is to heal a buttload on a boss you have to throw out every nuke you have, and those nukes cause a buttload of aggro, so you will most likely pull aggro from your tank unless all you are using is mind flay and shadow word pain. You have to hold back alot to make sure the tank will keep control of the mob. I dunno about spriest x4 but i would think it would be the same way.