I have a 4K TV connected by a 4K FireTV and have 4K content (x264) that always defaults to 1080p.
My understanding is that Plex are yet to allow resolution switching to 4K as of yet.
It’s a shame, because KODI supports 4K and running the Plex App through Kodi allows for 4K playback perfectly, although in my case does Transcode. I’m guessing limitations on other underlying factors, but its the only way I have been able to enjoy 4K content until the native Plex Android App is updated to support 4K.
Plex slacking behind in this area at the moment in my opinion.
Anyone feel free to correct me if I am wrong, but from what I’ve witnessed, the Android App does appear to support 4K at the moment.