I am having this issue with 4 - 6950s in a system. I even reinstalled windows from scratch to make sure it wasn't a driver or registry issue.
I checked all the video cards and they are fine. Also I do not have this problem when running 4 - 5830s in a system.
My last possibility is that the cards are pulling too much power from the PCI-E that the 4 card doesn't work.
Last solution I will try is to get a pcie to pcie extender with molex to take some load off the motherboard, but this doesn't come in util next week.
Let me know if you solve the problem.
BTW, power supply is not an issue, I have a 1200watt antec