The semifinal Story Driver in "Back to the Future" (the one that ends the third signpost) happens when George stands up to and punches out Biff. That leaves Biff out of commission for the entire fourth signpost (minus the epilogue where he's waxing the McFlys' car). So who fulfills his OS character role?
Looking back at my reply on @mlucas's "The Fourth IC Signpost in a Steadfast-MC Story" topic:
During the final signpost, Biff's OS role seems to be handed-off to the guy who takes away Lorraine from George! He acts the same way that Biff would if he were there, and therefore fills in the gap created by Biff's absence. To drive the point home, this guy's creepy laugh while dancing with Lorraine even sounds like Biff's own laugh.
On a side note, it appears that this scene touches on at least three of the four throughlines, with Marty quickly fading from existence, George's hesitation to take Lorraine back, and the aforementioned guy playing out Biff's role. Looks like a multi-appreciation moment!