Depends on whether you mean horizons or not, but taking the worst case scenario (at a big base on a planet with loads of AI and cmdrs around), to maintain 60fps at 4k with everything set to maximum, you'll need a 1080ti.
If you don't mind the occasional drop to mid 50s in extreme situations, then a vega64 or gtx1080 will do just fine.
If you're not running horizons, then a 1070/vega56 will do the job.
CPU is largely irrelevant, pretty much min spec will do the job.