@TeknoJunky
Am I right in thinking that merging 4K and 1080p in the same library will require the “split apart” (for duplicates) thing if using restriction labels? (Or different agents… which would be a shame with the new Plex scanner agent.)
But that question aside, going back some time there was a lot of chatter from Plex about the server/clients becoming smarter regarding the choice of file for the client scenario. From what I recall back then the general consensus was that in actual fact Plex would choose the most unsuitable option and take great pride in transcoding the 4K HDR copy of a movie to cellular mobile whilst pretending the 1080/720p didn’t exist.
Well at work last night I decided to test this again.
I added a whole host of 1080p duplicate versions to a 4K HDR library. I watched in PlexWeb as all the 4K versions switch to the 1080p as default.
I then loaded several movies on my non-HDR iOS mobile (which still showed the 4K version as default.) Despite that Tautulli and PlexDash showed that it was always transcoding the 1080p version and not the 4K.
So then the big test was when I got home. Which version would it play on a 4K TV via the ATV 4K and Shield 2019? In every case with the same 6 movies it always chose the 4K and DV/HDR was triggered.
As one final test, the spare room TV with a 4K Roku stick connected to a non-HDR 1080p display (with bandwidth limits set) it always again chose to transcode the 1080p.
Right now it seems that Plex does indeed make the right choices…at least on iOS, Roku, tvOS and Android TV.
Admittedly this was all just tinkering and what happens in the real world with several users on various clients is yet to be tested…But It does make me wanna go ahead and test it.
Also just to add that although PlexWeb changed the default to the 1080p file, not a single other mentioned client did. Despite that, each client made the most logical choice depending on its capabilities. 
