www.makemkv.com

Since most 4k ultra HD movies come with limited additional audio or video, I'm wondering if anyone has setup a workflow of ripping the blue ray that is often included, pulling things like the audio commentary off that, and sticking it onto the 4K movie?

How does one go about pulling some audio tracks out of the blueray MKV, and moving them into the UltraHD MKV?

The MKVToolNix tools are probably a good place to start.

On my Plex server, I have a separate library for 4K movies. When I get a disc with both a regular blu-ray and a UHD, I just rip both and put both on my Plex in their respective libraries. Almost always this results in just the movie in the 4K library and the movie + commentaries + blu-ray extras in the other library. If I'm listening to a commentary, I generally don't mind that I'm 'only' watching a 1080p version of the movie.