I am looking at a EVGA Z390 Dark motherboard, as well as the Intel i9 9900k CPU. I am having difficulty selecting a compatible RAM stick for it. The motherboard supports DDR4 4600MHz and the CPU shows only DDR4 2666MHz. Am I completely limited to 2666MHz RAM, or can I go with something higher? If I am limited, why is all the 300 series Intel processors only able to support a 2666MHz at most when the 300 series motherboards can do nearly double that?