iMovie reducing video quality

Try a new project and put your HD into it as the first clip added. See if it displays properly there.

Also, the editing display is a medium res one for editing purposes. The shared out project likely will be full quality.

You said that you were using the built in screen capture and that it was not HD. If it is not HD it won't be HD in iMovie either.

-- Rich

At this point you might consider contacting Apple Support and talking to the savvy techs there to see if they can solve your issue. Unlike on this forum the support people can remotely access your computer. To get there, click on the Contact Support item in the upper right hand corner of this forum's screen. Navigate to a place where you can input your telephone number.

If you have time, post back with your results, as it might help others who are experiencing the same problem that you are.

-- Rich

Sounds like your first clip in a project is not HD. May be you changed monitor resolution? Or something wrong is with screenshot app? Anyway, try to open your fist clip in QuickTime. Inspector window will show your video clip quality.

Try deleting preferences. If that doesn't cure it, try redownloading iMovie.

Deleting Preferences

To delete preferences open iMovie while holding down the Option and Command keys and select 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 you get a 4k share option for your 4k clip.

Updating iMovie

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder. The redownload should be successful, however. Open your project and see if you can get a 4k share option.

-- Rich