That link has quite a bit of info. It doesn't tell me if I can pull audio off a Blu-ray disc and put it on the 4K MKV file though.
Sure you can....sometimes though, it's not as easy as just a simple remux. If the video differs by a few seconds you have to either delay/expand or even trim the audio file.
You would have to rip the disc with MakeMkv and then mux out the desired audio track.
Play the file you want to mux into with MPC and select the new audio file and see if they sync up.
Sometimes they do...sometimes they don't.
I've done it many times.
Bottom line....it's not worth doing for every disc, just the big offenders.
Lots of file freaks like to mux new Atmos files into older files...same principle as above applies.
Sometimes it's easy...sometimes its not.
When you get into trimming a few seconds off the start of a 40 gig or so file hoping it syncs, you get into mass amounts of file manipulation.