Tested new mpv version with 4K movie content (Blade Runner) with my i7-2600 and GTX 1070 with 48fps and lowest quality settings in SVP and got SVP Index between 0,8 and 0,9. Before new mpv version i had SVP Index between 0,7 and 0,8. It was improved but still not playable for me 
i know my cpu isnt good enough. At least I can play 4K content (Also only with 48fps and lowest quality settings in SVP) with mpc-hc but not with HDR (ffdshow doesnt support HDR yet).
Still, for me the mpc-qt has better (best) performance with 4K content than the mpv default player. If you enable hadware-accelerated decoding and CrystalHD/CUDA hardware decoding backend in mpc-qt you get some more performance stability. In some sences of the same 4K movie (Also Blade Runner for this test) I get smooth 48fps, but in other scenes it stutter again. But care, you need a high end gpu for that settings in mpc-qt.
Well, I think the only chance to play 4K content is to buy newer CPU, or wait for further versions with improved performance.