In the latter case, the delay window is up to 4 hours, which seems unlikely barring some natural catastrophe It's unlikely that one flight will be over 4 hours

In the latter case, the delay window is up to 4 hours, which seems unlikely barring some natural catastrophe It's unlikely that one flight will be over 4 hours late; but the delay time which matters is based on when you reach your final destination. For example, a few years back I was flying LHR-YUL-YVR; the flight from London to Montreal was only 1.5 hour delayed, but that was enough that I missed the last flight of the day from Montreal to Vancouver, and qualified for compensation based on my arrival in Vancouver being 10 hours late.