Difference between 4x 5870 and 2x 5970, power consumption. Almost 200w difference, even if the 5970's core will be OC to 5870 core speed the difference is still big. If your planning 4 cards on 1 motherboard, you should consider water cooling the cards.
Yeah, after looking at the numbers more, I decided that getting 2x 5970s does make more sense. IF I can get them cheaply enough.