Downscaling 4k to 1080p in Premiere Pro, Image Zooming 2x
Changing the preference doesn't change any clip settings retroactively.  But once you have set it, and then create a new project and a new sequence, and when you freshly import your 4K clip and drop that freshly imported clip onto your freshly created sequence in your freshly created project, it should work as you expect.  And if you right click on the clip, it should have a checkmark next to "Scale to Frame Size".