If I was set on getting a radeon rx 5700 xt for the GPU. and want to take advantage of the PCIe4, I have to go with an AMD processor based motherboard? Is this correct?
Pretty much the same answer as above. Yes you are correct AMD 3rd gen processors are currently the only ones that support PCIe 4. At the moment PCIe 4 doesn't have significantly better performance than PCIe 3 but that will undoubtedly change as the technology gets better.