I went looking for the answer to the question about Google Play 4K for the Shield on the NVIDIA forums - it doesn't look good.
https://forums.geforce.com/default/topic/980553/shield-android-tv/google-play-movies-4k/
Google Play 4K requires VP9 Profile 2 and the Shield only supports VP9 Profile 0. There is a comment made at the end of that forum thread about NVIDIA solving this via their programmable GPU, but I think that is nothing but a stretch.
This doesn't end my love for the Shield TV, I will continue to use them for quite some time. If the new Shield comes next year and offers support, I'll buy one immediately. I may not have interest in buying $30 UHD movies, but rentals are quite a bit less than that.
http://www.androidheadlines.com/2016/12/nvidia-shield-will-be-updated-to-support-google-4k-videos.html