4K hasn't rolled out to my account yet so I can't test this myself, but all of the articles that mentioned compatible devices omitted mention of the Shield.
I can't find it now of course, but there was a reply to a post somewhere here in Reddit in regards to the supposed Shield TV 2 that said something like "hopefully that one will be compatible with Google Play 4K" or something like that.
Are there any Shield owners with a 4K TV that can shed any light on this? It seems strange to me if this true. The Shield may be two years old, but it is the most powerful and capable box available by a longshot. Seems weird if Google is really shutting us out.
The Shield can play 4k, so it's probably just an omission in the app until it is updated.