Some, but not all, 8-bit h.265 4K hcv1 mp4 HVEC should work fine in QuickTime on that machine.
hev1 flagged mp4 HVEC files won't work.
10-bit files will stutter.
mkv files won't work
If you're trying to play a 10-bit file, you're out of luck, because your machine is not supported by Apple for 10-bit 4K HEVC hardware playback.