4x 7990 setup problem

Hello,

I have a problem with the Asrock H81 Pro BTC when trying to plug 4x 7990 cards (8 GPUs). My setup:
- Intel i5 4460
- 2000W PSU (Platinum)
- 16GB RAM
- 60GB SSD
- 1x to 16x USB powered risers

If I only plug 3 cards it works flawlessly. Whenever I plug the 4th card the computer doesn't POST and stay there...

I tried to use the 16x PCIE slot, different combinations of 1x PCIE slots, with/without molex on MB, disabled all unnecessary (audio, intel virtualization, etc) in BIOS, set PCIE speed Gen1 and Gen2 in BIOS, boot without RAM, etc.

Could it be that my CPU cannot provide enough PCIE lanes?

Thanks guys.