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.