6990s are a stupid buy for mining anyway. $729.99 + 8.50 shipping for ~800Mhash/sec? .93 $/Mhash, or 1.08MHash/$.
Even if you manage to cram 4 into a computer somehow (which is as I understand it not really feasible) you're pulling less than 1Mhash/$, and generating $.017/MHash (per day).
If you buy it for a casual mining/gaming machine, it's still a crappy buy as 2x 6970s are cheaper and run better, but I'm not here to argue that I guess.
You're not counting in other costs:
4x6990 = 8 GPUS, you only need one computer if the mobo supports 4 pci-e slots.
8x6970 = 8 GPUS, you need two computers = two motherboards, power supplies, processors, RAM, casing, possible WLAN adapters etc.
It's actually more or less a bargain plus it saves space.