vu64, Outside of the reported clocking issue on the 2nd chip of the 5970, I'm not aware of any issues with sticking 4 GPU's in a machine as long as the Motherboard/Chipset support it. This would give you 8 devices if they were the dual chip cards. I have a machine with three GPU's in it, but I don't have any 5970's or 4870x2's to test it myself for more than 4 devices.
vu64, Most of the dev's are on vacation until the new year, and I don't have the cards to test in my machine. The problem zpdixon found has been reported, but that shouldn't stop from a machine from being able to use 4 5970's.