With this config you should be able to run those cards, but the problem could be either something with Windows when connecting the 4th card. I believe I have seen posts on this.
But, it also could be a heat issue.If the cards are not breathing properly, then they are overheating, and your mining is getting shut down.
Hi,
Thought in first time about high temperatures, but XFX7950 its in design with high temperatures. I thought put on then water block but other mining rigs owners run this cards without problems and same setup. And other owners run more than 4 cards. Maybe this could be risers problems? Maybe I need change risers into powered?