Good movie mp4 metadata editor?

You can use Wwidd for video file and

EasyTag (for mp4 + audio files). To install EasyTag type:

sudo apt-get install easytag

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.