UHD is just another name for 4k.
no one is using VLC because there are a ton of better options out there
take a look at plex or infuse.
Plex runs a server program on your computer
Infuse accesses network shares directly on the aTV.
Both give similar experiences, Plex is a bit more full featured
Infuse requires a paid upgrade to play certain files
Plex will play everything for free - There is a paid upgrade to add features.
Plex will re-mux your MKV files into something the aTV will play, (re-mux takes the video and audio streams and puts them in a new container, it leaves the streams completely unaltered)
Plex will also convert any files as needed, so the 4K-HDR movie you have to watch in the living room, will be converted to 1080-SDR for watching on a different TV that doesn't support 4K
Re-muxing doesn't take much processor power on the server, converting can take quite a bit.
They will both play 4k and HDR content,
(You can only get Dolby Vision when streaming, but regular HDR will play)
as others have said
Netflix, prime, disney+, aTV+ --- all have at least some of their shows in 4k. Some of those are either HDR or Dolby Vision.
Showtime is starting to have 4k, there are a few shows and some of those are HDR.