This question is too techy to hope for an answer on here, never mind a reliable one. It's probably a better idea that you do some research on gaming in 4k in general.

AFAIK, the recommended requirements for DOS2 and The Witcher 3 both list a GTX 770. Now if you refer to this article Witcher 3 Video Card Benchmark, then apparently a 2x GTX 980 SLI would give you 42 FPS avg. on Ultra in TW3.

This could be used as a rough starting point, I suppose. Next you could try comparing the benchmark results for a number of high-end rigs to narrow down your options. Something that might also be relevant is that the Definitive Edition runs more smoothly than classic on the same rig.

Last edited by Try2Handing; 15/11/18 02:24 AM.
"We make our choices and take what comes and the rest is void."