Let's assume they manage to resign Tobey & the crew for another film. Wouldn't it make sense to film 2 or 3 films back to back while Tobey can still be convincing in the role? He is getting old. I'd hate to see them start a whole new storyline in part 4 and then watch Tobey walk in part 5. There's little risk since the movies all make money so just film 2 or 3 back to back which will give you plenty of time afterwords to starting looking for fresh blood.