Yes, "4K" content is supported, depending on what model Roku player. If your Roku model doesn't support 4K playback or your content is not compatible, your Plex Media Server will need to be capable of transcoding the content.
The Roku 4 supports native playback of 4K content, so long as it meets the following criteria:
- Resolution: 3840x2160 or smaller
- Video Encoding:
H.265(HEVC; both 8-bit and 10-bit) or
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.
Other Roku Models
Aside from the Roku 4 player, no other Roku models support 4K playback of content. So, on other models, any 4K content you have would need to be transcoded down by your Plex Media Server.
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.