According to my (albeit limited) understanding of the theory, change in either the MC or IC can be gradual or sudden, or at times can be more gradually while at others more sudden, but most of the time if there’s no more growth in the character’s resolve during that fourth signpost, what’s the point of that final signpost?
I believe Back to the Future is actually a strong example of this. While George McFly has gone very far down the path towards change by the beginning of the fourth “act,” as illustrated by finally standing up to and punching Biff out, not long afterwards, when another guy takes away Lorraine to dance with her, George politely takes her back, but only after a brief hesitation. George’s final signpost clearly strengthens Marty’s resolve, as he takes over for the injured guitar-man, which in turn helps George to put the final nail in the coffin of his old resolve.