My money would be on lack of power.
Forgot to mention, I use a Corsair AX1200. Three cards run at 850W with slight undervolts. This should be fine for the 4 cards.
From your own measurments..
(850W/3)x4=1133W
1133W/12V = 94.4A
How many amps are on the 12v rail? 100.4? Leaves 10A. Not much wiggle room for something else on the motherboard demanding more power.
My measurements would have been..
8-Pin = 150W
6-Pin = 75W
PCI-E Slot = 75W
Total = 300W
x4 = 1200W
CPU/Memory/HDD = 100W
Grand Total = 1300W
12V Rail is indeed 100.4A.
The 850W is the total system usage though, so with a 4th card it still would be below 1133W, probably around 1100W. Idle usage without any videocards is around 80W, but let's say 100W.
(750W/3)x4= 1000W + 100W = 1100W
1100W/12V = 91.7A
Still not much wiggle room, but it really should be enough. I see tons of people run 4 x 280X on 1200W supplys. This has a gold certification as well, and the 1100W is actual power from the outlet, the actual PSU usage should be quite a bit lower.
Thanks for the reply.
Not sure about the socket, anyway to test this?
1. Use a 1X riser cable on one of your known good cards, to verify that the cable is OK.
2. Take that assembly and stick it into the 1X socket.
I'd do this with 3 cards, ie, a known working configuration, as I'd hold the other variables constant.
I have done this already at the beginning, with the same result unfortunately.
Thanks for the reply's again.