User blog comment:CureLightning/Part 5 announced??/@comment-25446207-20161228134449/@comment-3487190-20161229115147

@Ruruzuzu ? Miki is a senior and she shall not register for this event. Remember that it was the Yuzu beat down all seniors to become S4 at first. So in this case, it should be that Saya can challenge Yuzu again.

I would sorta agree with you, but we also have to go with the rule that Aikatsu! has to be about the main characters who have spots in the arcade game, and unfortunately some of the seniors are only background characters for the anime.

Inevitably there has to be one youngster amoung the next S4 team, just like Yuzu was. But I would doubt that there are gonna be more than one rookie idols winning which seems very illogical.