4K can be problematic, there is a thread that’s probably worth reading.
Without knowing the details of the 4K file it is difficult to know, as there are so many variables, data rate, audio stream etc
fwiw VLC is probably a more capable player than Exo Player which afaik is what plex uses.
In the settings on the Android app I would turn on Display Information. this will show you a bit more info about what plex is doing, it may be trying to transcode the audio.
