The other 3 girls could make a lot of stuff without Haruna during this period, like creating new songs or appearing on radio and TV shows, but I think it's fair for them to take a rest. And most probably go watch Haruna on stage.
And who knows, this first step in acting may open a whole new world for SCANDAL, like Haruna herself and all of the others to appear in movies or dramas, so the best of luck for her.