I just thought of something what if there is a book 5? I feel it would be a bit weird if all the problems were fixed in book 4 and every is just wrapped up like that. Maybe book 5 could simply be an epilogue where you go back to your time with the girls, or maybe it could be you going back to the "present" with the girls to fix your world. I don't know I feel like its going to be tough to wrap up the story without the girls all being present together. I would like to also point out one thing notice how each girl in the screen at the top of this page represents one book but notice the MC is also present in that picture, sure it could simply be to show the MC but who knows.