streaming 4k from amazon/disney/etc, or playing a file locally from usb, or using kodi/vlc, are all entirely different than plex.
the first thing you should check is your plex server dashboard, and see if it is transcoding.
most likely it is, and the problem is probably the audio and/or the subtitles.
you may find some more useful info, like how to find out why plex is transcoding, and what is causing it, @ [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
