for any number of reasons, the stream is switching from direct play to transcoding down to 1080 (thus loosing both 4k and HDR).
the logs indicate that you do not have hardware assisted transcoding, so that causes your cpu to convert the 4k, and which is extremely process intensive and your server cannot keep up.
most smart tvs cannot direct play HD audio which is the typical culprit.
subtitles also can cause problems and make plex transcode.
you can make sure subtitles are disabled, and choose a different audio stream (ie dolby digital/dtx 5.1).
this may or may not be helpful @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
