Converting mp4 files for iMovie

I had some 8mm film digitized by iMemories and they came back as mp4 files. I wanted to edit them in iMovie, but it won't recognize the type of mp4 file.

I read a previous post (2015) with suggestions about this problem and I tried the suggestions but didn't have any luck. Here's what I tried: Using iTunes to save a new copy of the video; using Quicktime to do the same thing; uploading to youtube and downloading. None of these things worked.

Have there been any new developments on how to convert these files to something that will work in iMovie?

Thanks for your help!

Diane