You need to give more information. 264 is just one of three parts of the file. anyone of which may cause the issue. But I will say that usually it has something to do with the subtitles.
Get on Plex and find out which type of audio and video codecs are used, and/or try watching without subtitles.
If you start the 4k video, and go to the dashboard on the Plex server it will tell you if you are direct playing, direct streaming, or transcoding. Chances are you are transcoding or streaming the video. If that is the case then you are having network issues due to the tvs limitations.
There are ways to work around this if it is the case.