Good movie mp4 metadata editor?

None of the other solutions proposed here worked with my QuickTime MOV file (a movie recorded on an iPhone and edited with the iOS iMovie app). VLC almost worked but the 'Save metadata' button doesn't save anything. exiftool, however, did work!

apt install exiftool
exiftool -overwrite_original -year=2020 -comment="City trip to Paris" -title="Paris" '/tmp/paris.mov' 

Tested in Ubuntu 20.04.

AtomicParsley command line tool can set all the .mp4 metadata tags (supports 3 tag schemes: iTunes-style, 3GPP & ISO), including artwork and long description and even custom tags (I don't know what's this, never used it).
To install:

sudo apt install atomicparsley

To use:

AtomicParsley --help

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gtkpod is a GUI application that uses an atomicparsley library to set some metadata tags. To work on the .mp4 files you need to turn on the MP4 Plugin in the Preferences.

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EasyTag is another good GUI application that can set the .mp4 metadata tags. It's more easy to use than gtkpod, but latter can set more tags.

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Kid3 is also a very good GUI application that, like EasyTag, can edit batch of files.

I usually use ExFalso sister to Quod Libet music player

You can use VLC. Simply open the file and hit CTRL + L to show the playlist or from the View menu choose Playlist.

When you see the list right-click on the list item you need to edit and click on Information

If you're just trying to see the metadata, exiftool gives pretty extensive metadata on most media files, including .mp4/.m4v files.

If you need to edit specifically .mp4/.m4v files, try atomicparsley ( http://atomicparsley.sourceforge.net/ ). It can look a bit daunting at first, but a few minutes with the help section, and you'll get the basics.