My plex server is on a PC (i5-7200U CPU @ 2.50GHz 2.70 GHz) 8GB RAM.
When I try to play 4k UHDBlu Ray rips (mkv) files, the player (Roku ultra) buffers terribly and finally gives message that server is not powerful enough to process the playback.
I have Plex Pass, but will unsubscribe if I can’t play 4k rips.
Roku Ultra is hardwired to the PC (Plex server) - not wifi.
If by chance you are trying to play an HDR rip on a SDR display, that asks the server to do tone mapping. A Windows Plex server cannot take advantage of Quick Sync tone mapping either.
Maybe that’s not your setup, but it would explain it.