At 4k, you are 100% GPU bound. As stated above, the CPU doesn't really matter, just get a modern one. I would look at a 6-core one just in case that games actually make better use of cores in the near future, though I don't expect that to go mainstream very soon and if you don't mind swapping, a 12100 would be a decent and cheap pick, too. It's your decision. I wouldn't spend too much unless you plan to do other things like code compiling, though.