I have tried to play a 4K movie on my P55-f1 from the Play Movies And TV app and cant get it to display in 4K HDR. I have tried casting to my tv, playing through the app on mibox, casting to mibox, playing on youtube on mibox, playing through smart youtube app, and all the same on my ADT-2 device.
Do you have Netflix? does the test patterns video on Netflix show 4k?
I have this tv and a Shield TV plugged into HDMI 2. Google Play is displaying Into the Spiderverse in 4k. Double chekc the hdmi ports you are using, they don't all support 4k.
Google Play Movies has a select device that can stream in 4K HDR.
The functionality comes and goes. It has to do with the google cast version part of smartcast os of the vizio tv. (search this forum and you will see all the problems: https://www.reddit.com/r/VIZIO_Official/ )
As of right now I am pretty sure it broken for everyone. It has worked in the past, so no telling when it will come back. Sorry.
As for it not working on the mi box, not sure. I will have to check that as I have the mi box s. I am pretty sure it can play HDR on the mi box s with the movies anywhere app. So if you link your google play movies account to movies anywhere and play it through movies anymore, maybe that would be a good workaround?
Not sure if it is regarding your problem, but here where I live Google Play doesn't support 4k and/or HDR