Most of my 4k HEVC H.265 videos play fine in direct play mode in Plex, as long as they are 30 FPS. Some of the 4k videos I have in this format have decryption errors with green tiles and 60 FPS videos transcode.
Most 4k H.265 videos I have don't have problems with decryption and are 30 FPS and they all play very smoothly with no dropped frames BUT I am only running the Mi Box on a 1080p display. That might have something to do with it.
Plex shows direct play is happening and that the resolution is UHD.
Not trying to be a smartass but why bother with 4k videos on a 1080p display?