I have an update. I've disabled the intel virtualization technology and hd audio. I've changed the top of lower usable Dram from dynamic to 1gb, and PCIE link speed from auto to gen3 for all pcie.
After that I have managed to connect 3 graphics cards.
However, when I put the 4th one (the last graphics card I have) the computer doesn't switch on. I have just a black screen and a b2 code on the motherboard. How can I solve this?
Make sure you have enough PSU power rating watts to supply your 4 GPU's, What are your GPU types? check also the 4th GPU if its connected correctly thru powered riser and your PSU.
much better if you could tell us your PSU power rating and your GPU types