Season 4 was mapped out and ready to go after season 3 wrapped, then Disney decided they needed a Mando movie, so Favreau and Filoni took the overall story arc of Season 4 and turned it into the script for the movie instead.
Season 4 isn't happening in that form anyway. I would assume if the movie is a success it will get movie sequels rather than more TV seasons, and if it isn't successful it will just be put aside.