Try setting PCIe link speed in the BIOS to GEN1. (Should that fail, try the other available options with this setting.) My guess is that PCIe lanes is your issue, since your system will likely default to having one card run in x8 mode, and 2 others run in x4 mode, causing problems for the fourth card.


Should that fail...
Clear CMOS (use the jumper or remove the CMOS battery for about 10 seconds)
Reseat CPU
Reseat RAM