That’s the problem you run with fantasy. After a certain point, you’ve used all the tricks of the trade such as memory losses, witch fights, and even walking brooms. I think this show probably needs to come to a close at the end of S4. I think once the author of the book is revealed (assuming there is one) and the sorcerer’s identity confirmed, then that would be the appropriate end point for the show.
Don’t get your hopes up. Once is still doing well enough for ABC, and remains as one of their top rated shows.