A high-resolution clip added late in an edit does not necessarily change the format of the iMovie project. On Mac, the first clip added establishes the project resolution and frame rate. This makes the beginning of project creation worth checking before spending time on titles, cuts and narration.

Apple documents 4K projects at 30 frames per second and higher frame rates at 1080p or 720p. Its sharing guide describes 4K UHD output at 3840 by 2160 and 30 fps, and 60 fps sharing at 1080p or lower. Treat resolution and frame rate as separate requirements.

Inspect the project before changing the export

Open the project and click Settings above the timeline. Read the format under Project Settings. Compare it with the intended delivery: a request for 4K detail is different from a request for 60 fps motion.

For a new project, add suitable 4K footage first when establishing a 4K project, or suitable 1080p or 720p 60 fps footage first for a 60 fps project. Check the displayed format immediately. Do not assume moving a clip to the front of an already edited timeline repeats project creation.

If the existing project is unsuitable, preserve it and make a small new test project with the intended first clip. Prove the format and export choices there before deciding how much editing work needs to be recreated.

Use a short export as the acceptance test

Share the test through File and inspect the Resolution choices. Apple documents 4K and 1080p 60 options for their corresponding projects. Save a clearly named test file.

Play a passage containing movement and another containing fine detail. Check the saved file’s reported dimensions and frame rate in a suitable media-information view. These observations establish what was delivered; a filename containing 4K does not.

Keep the format decision with the project notes. Adding ordinary footage to a higher-format project does not establish that the source itself contains more detail, so inspect the weakest important clip as well as the best one.

References: Apple: iMovie project creation, Apple: 4K and 60 fps in iMovie.