User blog comment:AriusKill/A Possible New Brand In Aikatsu Stars/@comment-28417557-20161122090422/@comment-3487190-20161122125100

Not knowing how the story goes, but by the fact that Lily is a senior and she would eventually leave earlier, Yume won't have to fight her in the last year of her school life, so I don't think Lily's dream definitely conflicts with Yume's.

But like you say, probably Lily just finds another chance to do a brand without becoming an S4 so that explains the timing issue.