Yes, "4K" content is supported for playback on Xbox One devices, so long as it is compatible (see below). If the content is not compatible, your Plex Media Server will need to be capable of transcoding the content.
Compatible 4K Media
Native playback of 4K content is supported so long as it meets the following criteria:
- Resolution: 4096x2160 or smaller
- Video Encoding:
H.265(HEVC; both 8-bit and 10-bit)
- The content is being Direct Played
- No subtitles are enabled
Tip!: If you have 4K content with a different encoding (such as H.264), it cannot be played at 4K and will instead be transcoded down to a maximum of 1080p.
Playback of 4K Media
Only the Xbox One S model is capable of playing content at a 4K output resolution.
Other Xbox One models can play 4K content (so long as it is compatible, as noted above), but the content will be downscaled by the console to 1080p output.
4K content that is not compatible requires transcoding. Note that transcoding 4K content can be a CPU-intensive process and some lower-powered computers (and especially NAS devices) might not be capable of doing so in real-time.