What is one responsibility of the No1 if the No4 is not on board during the PDIs?

The responsibility of the No1 in the absence of the No4 during Pre-Departure Inspections (PDIs) is indeed to conduct the No4's tasks. This is crucial for maintaining the safety and operational integrity of the flight. The No1, typically the lead cabin crew member, must ensure that all required checks and duties usually assigned to the No4 are completed. This not only involves adhering to safety protocols but also ensuring that all cabin preparations meet regulatory and airline standards.

This responsibility is integral to ensuring that the flight operates smoothly and that all safety measures are in place, as the absence of any crew member can create gaps in essential safety checks. Therefore, it is important for the No1 to take on these tasks, ensuring that the cabin crew functions effectively as a unit, even with changes in personnel. The other options do not recognize the critical nature of task completion in absence of a crew member, thus failing to capture the importance of the No1's role in maintaining safety and compliance.