I assume you are going to use the pc for gaming, but please feel free to say what you are going to use the pc for.
If you want 4k 60fps gaming then you need a gpu from nvidia. I would recommend you the 1080 ti for gaming. But my 1080 ti can't run all games on 4k on 60fps, games like Gta V, Dying Light and other gpu heavy games starts stuttering and you don't get more than 40-50fps, i would recommend you buying 2 1080 ti's i know it's expensive, but it's retty much your only option.
Hope this answers your question.