Finally, I found a free, fast and easy tool to tag MPEG-4 files: MetaZ.
https://github.com/griff/metaz
It’s pretty easy to use. One little detail to be mentioned though:
You can add chapter marks by pasting them in the following format:
00:00:00.000 Intro
00:00:30.000 Antihelden
00:5:00.000 Die Jungs Aus Dem Reihenhaus
00:15:00.000 Fenster Zum Berg
00:20:00.000 Affentanz
00:22:00.000 Manfred Mustermann
00:36:00.000 Solala
00:40:00.000 Safari
00:50:00.000 Liebe & Hass
00:55:45.000 6 Meter 90
01:00:00.000 Blattgold Auf Anthrazit