well that depends entirely on the tv, did you read the FAQ thread linked above?
it provides a whole ton of info and discussion on 4k usage.
my guess is simply that you are trying to play truehd audio and/or subtitles, and its causing the server to transcode, and your hardware isn’t new/powerful enough.
if reading the thread doesn’t help you figure out the problem, then we need some clean logs from during when the problem occurs.
make sure DEBUG server logs are enabled
make sure VERBOS server logs are DISABLED

- restart plex server (so startup logs are included)
- wait ~2-3 minutes for server to settle
- play a video that you are having problem with
- if it plays, let it play about 60 seconds, then stop it
- wait ~30 seconds after stopping
- go to Plex Web > settings > troubleshooting > download logs
- drag/drop the whole zip into a reply
that should get us started, but if that doesn’t help figure it out, then we may also need the client logs.
client log info
server log info