Here’s how I now find if a movie is in 4K on Peacock: I use the JustWatch web site and look for Peacock/AppleTV bundle under 4K.
So far every movie that I’ve found listed as available in the AppleTV/Peacock bundle has been in 4K on the Peacock site.
Ex: The Mummy (1999)
https://www.justwatch.com/us/movie/the-mummy-1999
Use GRID view, go to the 4K filter, look for AppleTV/Peacock Bundle
Confirm in SF/Peacock

Notes:
- Above is for US, just change country as needed if outside the US
- You can filter movies at JustWatch for the Peacock service, but Peacock is never listed under the 4K filter at JustWatch so looking for a movie with Peacock/AppleTV bundle is a workaround just to find quickly if it may be in 4K at Peacock itself.