What else can it be?Just because it supports 4000mhz in theory, doesn't mean it will work.
Don't OC to 4000
Ram:HyperX Predator Black 16GB 4000MHz DDR4 CL19 DIMM (Kit of 2) XMPMemory controller is in the CPU and it has to be able to work at that speed, it has to match what MB demands of it. If it doesn't then it will throw errors.
Which system is that anyway ?