I would stay way from PSUs with multiple 12V rails unless you know what you're getting into. This Corsair 850W (12V@70A) or 1050W (12V@87.5A) should be sufficient for 4 7950s and a low-power CPU. If you insist on that Bulldozer CPU, go ahead and pick up the 1050W. Unless my calculations are way off, the 87.5A on the 12V rail should be more than enough.

If you want to run all four cards on a single board, just get PCI-E 1x to 16x risers. It won't hurt your mining and makes choosing a board a lot easier as most high-end boards have 4+ total PCI-E slots. Heck, even mid-range boards have 4 PCI-E slots.