In the 'auto write tags based on file names' tab, check the 'advanced operation' check box. Then use the <trash:x> command to remove characters.
X = the number of characters to remove.
for example: <TRASH:4><Title> - <Artist>.mp3
removes the first 4 characters of the filename and fills out the Title and Artist tags. Character 5 of the file name becomes the first character of the title tag. Everything after the ' - ' becomes the Artist tag. Lots of combinations possible of course. When all the tags are filled out, you can do the opposite and rename the file name based on the tags. That way the 4 characters are out of the file name also.
Hmm, did you understand that? I barely do myself after re-reading this. Best I can do though
BTW, if something goes wrong, don't panic, there's an undo button

Frank