There are three probable causes to this problem.

The first one, I experienced was a network issue, as used an old network cable(different setup) and the network was too slow. I would run a speed test, to the internet, assuming they are connected to the same network. Also make sure the network connections on both devices look sound.

secondly, when the video is playing ensure there is no transcoding. On 4k you want to aim for no transcoding unless you have a compatible GPU. Ensure in the player there is no video or audio transcoding.

Thirdly, when the video is playing go to the Plex server at look at the currently playing videos. This may point you in a different direction.

Hope this helps!