Make iMovie Output Full-Resolution Video
The problem is that iMovie doesn’t let you set a project’s resolution manually—for that you seemingly need the $300 Final Cut Pro X—and on the Mac, it fixes the project’s resolution based on the first clip you insert.
This problem has stymied many people, and there are numerous YouTube videos suggesting that you create a new project and drop a high-resolution photo at the beginning of the timeline to set the project to a higher resolution. That trick didn’t work for me. But I figured out a different way of convincing iMovie to allow me to export a 4K file.
