xbox is not a very good plex client.
the best for 4k is nvidia shield, it is the ONLY mainstream streaming device to support truehd+atmos.
other good alternatives are ROKU 4k devices or perhaps appletv if you are an apple fan.
amazon fire devices can work, but are more limited (no dts and other limitations), and are more problematic (in my experience).
you may also want to read the whole thread @ Plex, 4k, transcoding, and you to learn about 4k plex in general, common pitfalls/problems/etc.
