I play 4 because I play with a friend a lot. That way we can do stuff together without I have to drop one.
Also, with the new group find system it won't be hard to find an other dps for heroics.

I'm not planning to use my team for PVP

I've been thinking of swapping the two shadowpriests out for hunters. (any input on this?)

I don't want shamans or druids or mages because I already played these.