To play 4K MKV on Apple TV 4K, one recommendation is to use Infuse 5. But that doesn't mean there isn't a solution though! Here we put together 2 possible solutions for you. They are currently working on getting 4K content to play when using MKV container as well. The Plex team 'forgot' to update the Apple TV profile to support direct streaming when playing 4K material in the MP4 container, as this is supported out of the box. Based on our testing and some guys' feedback, Plex server on Apple TV 4K does yet not support 4K. There is a hot topic on Apple and Plex forum: streaming 4K MKV to Apple TV 4K via Plex server is not working. I use PlexPy while the MKV video format is being streamed to the ATV 4K to determine whether or not transcoding is taking place.' When I do that, the Plex server perform transcoding. 'There is no way for me to either 'directplay' or 'directstream' a 4K MKV/H.265 media from my Synology Plex server to the Apple TV 4K Plex client (all my movies are stored in my Synology).

Stream 4K MKV/H.265 to Apple TV 4K via Plex Not Working- Solved

Apple tv 4k plex app