Hi,
iMovie does not knock down the resolution of an imported clip. So if you import a 4k clip it remains a 4k clip. However, if you put it into a 720 project you will not get a 4k share option and it will export at 720. A project's resolution is set at that of the first clip added to a newly created project. Adding a 720 clip as the first clip added to a newly created project sets the project at 720. If you delete the 720 clip and then add a 4k clip, the project still remains a 720 project. It will not share out as a 4k project.
If it is not working that way for you, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if your 4k clips will download and share at 4k in a 4k project.
-- Rich