Per this topic and my experience the reasons for the Error - Upload error 4 during torrent upload/submission at https://thepiratebay.org/session/ can be:

1. Too long Torrent name (or torrent filename - i doubt about file name).
2. Too many files in the torrent (too big torrent file - lowering count of pieces by making these bigger during torrent creation may help?)
3. torrent description you are trying to enter is too large
4. '&' in the title?

Source: https://pirates-forum.org/Thread-ThePirateBay-Error-Upload-error4