Because with 4, there is too much overhead, and ultimately CPU bound. To the OP, the HX850W should be more than enough. Why not go for 2x 4890s? I wouldn't be surprised if it's better overall than 3x 4870s.
yahh your right. the reason why i7 is soo amazing for cf and SLU.
yahh 2 4890s would seriously kick @$$ and be alot cheaper, and scales better too.definately go with a pair of them. and make sure to overclock to one ghz