Well the risers on the original 3 all work without issue. When I plugged the 4th riser and GPU in is when there was an issue however I don't think it's the riser. If I leave that one plugged in and unplug one of the original 3 risers the system works just fine.
All the risers use a 4 pin molex for what it's worth.
Go to the BIOS of the motherboard and change configuration of PCIE express slots from Auto to GEN1. That solves your problem in 99% of the cases where everything else is working correctly.
Also before trying that try to lower the power limit of your GPU-s to check if maybe you have a faulty PSU which does not give the needed power to them.
I had one problem with a 3 x RX 480 board in a Thermaltake 700w gold plus which should supposedly run them pretty well, well it didn't until I turn down the power limit to -60% and from there no problem anymore.