Could I have the link too please?4k Plex servers are a thing (not for live content).
The main issue will be your internet speed. Depending on how the host has encoded the file you will probably need an internet speed of 75-100 Mbps to direct play from the server. Less than that and you'll need to transcode, in which case you'd be better off with a remux Blu-ray file.
I'll PM you a link where you can find some 4k servers.