i thought that newer platform using the amd fx74 ...the mobo has 4 pci-e slots and 2 cpu slots ... the fx74 is half of a quad cord processing scheme ... so you must use 2 ..and again to my knowlege ..this setup utilizes 4 video cards.
Crossfiring 2 Dual Cards has been done just like the Nvidia using 2 dual cards but I can't find anything on a using 4 single board. That requires at least 2 PCI-e controllers to get 4 x16 slots. Other wise even using 4 x8 slots requires 2 PCI-e controllers. And nobody has mentioned how you would connect the 4 cards. The SLI and Crossfire bridges only support 2 cards.