An am3 socket motherboard having quad crossfire would be a problem.
On one (or even two on some boards) of the gpus you will get half of the performance because it would run at 4x speeds instead of the 8x or 16x speeds.
if you want quad crossfire please go with a motherboard that has pci express 3 and has atleast 4 of them.
i dont think you will find a pci express 3 motherboard that has an am3 socket, sorry
you will have to get a newer one.
on pci express 2 you need to have the gpus to run at 8x, atleast, to get full performance (unlikely to get 4 fast cards running full speed on pci exress 2.)
on pci express 3 you can have them running at 4x and you would get full (or almost full, negligible loss) performance.
if you want to run 4 cards on a pci express 2 you will need very old cards (one 7970 would be faster than all four of them combined so i dont recomend you get 4 old cards)
listen, a new budget cpu on a motherboard that has pci express 3, and has enough slots, will be your best bet.
instead of buying an old cpu(slow, power wasting, pci express 2 only) buy a new one (same speed, more effecient, pci express 3 support) for the same price and you could run 4 7970s on it.
Not quite true.... Im using AsRock 990FX Fatal1ty Professional MB which has three PCIE slots running at 16x, 16x, 4x and Im using three video cards for mining and have absolutely no problems at all. And I think that ASUS 990FX Chrosshair V has 5 PCIE slots and supports for 4 graphic cards.
. Anyways.