Hi, Jane,
Jim has taken you through the correct approach to addressing your issue. His advice is right on. In his last post he intimated that the problem may be with your video clip. I have found with some 4k samples downloaded from the internet that they only show and export at 1080 from iMovie. But when I take a 4K 30p video on my iPhone it imports into iMovie as a 4K project and also exports at 4k. So apparently not all 4k is the same. The literature that I have read indicates that iMovie supports 4k at 30 fps. I would think that should cover 24p as well, but let's find out.
If you have an iPhone 6s or later, try taking a test video at 4k 30p. Import that video into iMovie and see if you get correct resolution and export settings on your project. If you do, then you know that the problem is with your original clip. Changing the frame rate to 30fps might resolve it. I believe that you can do that with the free download, Handbrake. See the frame rate as "constant".
If your iPhone vid won't import into iMovie at 4k, then do a reinstall of iMovie 10.1.9. Drag the iMovie app (the app, not the Library) into the trash, but don't empty the trash. Then update iMovie 10.1.9 from the app store. You won't lose any of your projects. To ease your mind, you still have your old iMovie 10.1.9 app in the trash, that you can drag back out in case there is a problem with the reinstall.
BTW, You computer should be 2011 or later with at least 4GB memory and an i7 processor.
-- Rich