For 4K releases, if the source is film then chances are it's going to be full 4K. Modern movies with CGI tend to be rendered at 2K because, even with today's technology, to render all that CGI in native 4K is prohibitive, especially if they have to change light sources or other objects to re-render due to a mistake, et cetera. But a lot of films from the 90s and earlier have benefited, and the old Star Wars movies (not the prequels!) look incredible.
Here's a fun article of titles released:
https://4kmedia.org/real-or-fake-4k/