Apologies for the delayed response! Been a bit hectic with the holiday creeping up fast and other personal life matters.
I think the confusion you have stems from earlier builds where chapters 3 and 4 did not have a clean transition. The trigger was essentially, optional, which I changed as I went on to make things smoother. For clarity, chapter 3 ends when you formulate the plan to retrieve the logs and tools to rebuild the bridge. And chapter 4 ends once the bridge is successfully rebuilt.
Hope that helps!