HI, DensF1,
What is the frame rate of your 4k and 1080p clips? What is the frame rate of your project? What format are the clips? iPhone by default will record in HEVC (H.265) , that can sometimes be problematic. Open a clip in QuickTimePlayer and click on Window/Show Movie Inspector, to see the codec and frame rate of the clip.
Meanwhile, here are some other things you can try:
Deleting Preferences
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 the lagging and glitches disappear.
New Project
Another thing you can try is to copy your media into a newly created project and see if
that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the
media in your project and then Edit/Paste it into the newly created project.
Safe Mode
Try booting up in Safe Mode (see Finder menu for instructions) and,
after everything loads, immediately shut down and reboot in normal mode. Then
see if iMovie works properly.
-- Rich