see e.g. this thread: /t/11750/1
I am not sure if you got that right with the fields. The id3-org site shows you all the tag fields that are defined by the standard.
If the information you try to save is not covered by a tag field mentioned on the id3 org site then it is already covered by the option to create user-defined fields.
The "4 track" stuff is part of the TMED field - so that is already available in MP3tag.