Since the 4th movie wasn't especially well-received and certainly isn't well-remembered (and a studio isn't going to put their faith in Shia at this point anymore), they're hoping this one does well enough that it can serve as a "hand-off" of sorts to her character, to allow to get to do the crazy running and stunts and what not.

Will it work? No clue. And studios have tried to do this since the beginning of time with IP-- the old "hand it off to the kid/student/friend/etc" thing. And, like... I can't think of one time it's *actually* worked and the kid became the lead in several franchise sequels? Maybe I'm forgetting an obvious one.