What must be done if No.4 is not onboard during PDI according to regulations?

When No.4 is not onboard during the Pre-Departure Inspection (PDI), it is essential for No.1 to perform No.4's checks. This is a crucial safety and procedural step to ensure that all checks are completed before the flight departs. The PDI is designed to confirm that all crew members are present, prepared, and that their duties are fulfilled to maintain safety standards. The absence of a crew member necessitates that another qualified member, in this case, No.1, steps in to carry out the required checks to ensure compliance with regulations and readiness for the flight.

Options discussing delays, filing reports, or notifying ground control may pertain to operational considerations, but they do not directly address the immediate need to maintain safety protocols and ensure that all functions are adequately covered during the PDI. Thus, having No.1 perform No.4's checks is the appropriate response to guarantee that all regulatory requirements are met before takeoff.