Are you using only one PSU? If not, you may want to check if the secondary PSU is actually running, as you need to jumper it so it will turn on the video cards (common rookie mistake) - turn on the psu that is not linked to the mobo first.

If you did tried to swap between them and the mobo are not recognizing more than 3 cards you may want to check if your PCI-e are all set to x1, as many mobos have its pci-e sharing energy, so if you don't set it to x1 it won't work.

Another think that you may want to check is if your mobo have the molex slot (4 pins) to allow an additional input of energy. Many mobos that allows more than 4 cards have it and it's required to be filled with the molex cable from the PSU.

Check the TOLUD configuration of your mobo, it's no joke when you are mining that you need to do some tweaks at BIOS so it can work, do all steps and if possible turn OFF all non necessary stuff.