At least it does on both my Shields, I assume the same for all Android TV based devices. The problem is finding 4k content. There isn’t much, but it’s there- be nice if Peacock made it easier to find.
Edit- I noticed the latest build does not output HDR on my Shield. I installed build 3.9.23 and that one works.
I would like to know what you watch also to test.
Check the "New Movies from Theaters" section, clicked on on a few movies there and they reported "4K UHD" available. But I tested with the TiVo Stream4K and turned on TiVo's debug video-info-overlay to see if in-fact it was receiving 4K content, and both the The Black Phone and Minions: The Rise of Gru would only play in 1080p; this is more due to the TiVo being a shit SEI-Robotics device where individual apps don't have permission to change the HDMI mode let alone check what mode the HDMI connected device is already in.
I am having the same issue on my 75" Vizio PQX.. On the title screen it reads "4K UHD" on 2 different movies I watched tonight. One was "Beast". The other was "Halloween Ends". Midway through each film, I pressed the info button on my remote and the screen overlay read "1080p". I'm only having this issue with Peacock.
What did you watch, so I can check ?