I have a 1920 x 1800 project wth 4K interview footage in it.
Edited the footage into the timeline as is. I see the whole frame as shot. Which is fine.
I would have expected the Scale value of the 4K footage to be approx. 50% in the inspector, but that is not the case.
Anyway, I occasionally want to zoom in and reframe a shot (Thinking I have enough pixels to do so) but for some reason the enlargement (less than 200%) makes the shot blurry as if I was enlarging regular HD footage.
Rendering does not improve things.
I'm also editing 4k into a 1080p timeline, but scaling works as expected. Yes, the footage shows 100% and I have to scale above 100, but the image is still crisp.
1. Your project could have turned into a 4k timeline based on the first clip you edited. Check the inspector and see if your project is 1080p or 4k.
2. If you synced audio/video with a multicam or compound clip, that clip could be 1080p with 4k clips inside. Whatever the parent clip size is, that's what the project will see. So a 4k clip in a 1080p multicam will be treated as a 1080p clip. Check the inspector and make sure your clips are still 4k.
3. The scale parameter only changes the perceived size, but doesn't throw away pixels. So if you drop in 4k into a 1080p timeline and keep the default settings to fit, you can scale up above 100%.