All devices except the 4K apple tv receive 4K content transcoded and properly downscaled. Apple TV 4K shows a black screen and a progress bar that make it appear as if the content is being fast forwarded.
When I get a situation like that it’s usually because that particular HEVC format isn’t supported by tvOS. What Plex does is to convert the container (if necessary) to mp4 and then the client plays it as a mp4 HEVC 4K video. But if the received mp4 video isn’t compatible Plex sends it anyway and the client goes banana. This happens to me with most 4K HEVC demo videos I have downloaded.
You’d need to verify if with a video supposed to work (for example Planet Earth II in 4K HDR plays well… Try to find a “demo” ) you get no issues.
Another thing you may try is to disable the “Direct Play” option in the client. In that way the server should be forced to transcode it, hopefully still in 4K (I think it does, but with my server it goes slower than a slideshow). If you have the horsepower needed that could be an option.
I’ve tried this via two different Plex Media Server boxes. One an Ubuntu server running on a Dell PowerEdge T30 and the other running on Windows 10 (quad core Kaby Lake i5). Both of the react in exactly the same way.
Because they are basically the same server (well, assuming the version is identical). The platform running it doesn’t matter.