Imovie

You would need to drag the Mp4 video into an iMovie project timeline. You could edit it ther, but your editing options would be limited. You could do splits, add new clips, add titles, and add effects, but you couldn’t undo any previous editing that is embedded into the clip.

IMovie should automatically have saved your edits. What happened there?

— Rich

Hmmmm. That's odd. You might try looking in the iMovie backups folder and opening in a backup library to see if your projects contain all of your editing.

To get to the iMovie Backups folder, follow this file path from the Finder menu:

Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups

When you get there you will see a list of previous backups of your iMovie library.  Click on one dated just before your issue started.  iMovie will open in that library.   Navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact and contain your previous edits.

-- Rich