Would 4x 6970HD be good enough for Battlefield 3 maxed out?

Yes, but a waste of money. Take it from someone that has 4-way sli, the scaling doesn't even exist after 3 cards and even those are a waste. Imho you're best off getting a decent board and 2 of those rather than some crappy hacked board that can barely support 4.

Personally i think the gtx580's are better, i do custom PC builds for people and i had an ATI 6970 lined up against an nvidia GTX 580 and the lighting i saw on it was just amazing, if i ever have em again ill upload pictures.