The full 2016 Vizio 4k TV line up will support Google cast, all Sony 4k TVs run Android TV and the nvidia shield is arguably the best 4k streaming device.....So why does Google play movies still not support 4k. Why hasn't Google made it a priority when YouTube already supports 4k and higher.
Google I/O 2016 just announced Oct 4, 2016 - Google Play Movies Store to start to have 4K content available, along with their Google Chromecast (3rd Generation) with 4K output. Also including an Ethernet port on the power adapter. See keynote in the jump below.
Chromecast Ultra (Ad) - https://youtu.be/Dgxz0kZ2dp4
Google’s Pixel phone event in 10 minutes via The Verge - https://youtu.be/p1qHV6ReJLI?t=6m59s
madebygoogle (Full Google Keynote 1h 30min) via Google - https://youtu.be/q4y0KOeXViI?t=53m32s
The resolution you can provide the movie at can be a problem for content providers. I think the rights to obtaining the 4K version is different to the HD version.
Interstellar for example is only available in Full HD (or maybe even just HD) on Amazon Video and Hulu, where the only place you could get it in 4K was through UltraFlix, which had a rental option for the movie. It's not even available on UltraFlix anymore. Given how Amazon Video already provides 4K content, they surely would have made Interstellar available in 4K if they had the chance.
I might be wrong, I'm just making an educated guess through how Interstellar was distributed.
I would think that's up to the film studios to provide that. I'm sure Google is more than ready to accept a 4K source file to serve up. Like you said, YouTube has been doing 4K for a while now. This is probably something to do with contracts with the studios, maybe requiring some legal re-working. Streaming agreements are really fucked up. For instance, you can stream channels like Fox and NBC but they blackout NFL games because they don't have the rights.
They're probably working on it, but I would guess there are lots of licencing issues for them to work out. Content providers are probably going to push for much more provider-friendly licencing in order to get 4k video, which is the opposite of what Google wants.
4K Video? How about basic 5.1 audio? Digital Pro Logic II helps a bit but its silly that I should pay $4-8 to rent a movie and not be able to use my surround system.