What immediate action must No.4 take after completing cabin secure checks?

After completing cabin secure checks, No.4 must immediately take a seat to ensure their own safety and readiness in the event of an emergency. Sitting down allows the cabin crew member to be in a secure position, enabling them to quickly respond to any incidents that might occur during takeoff, landing, or while the aircraft is in flight. This step is critical for maintaining safety protocols and ensuring that all crew members are securely positioned so they can support the overall safety of the cabin.

The other options may appear relevant within the context of cabin duties but do not take precedence immediately following cabin secure checks. Checking the galley, preparing refreshments, or assisting passengers are tasks that can be managed after ensuring that all crew members are seated and secured, thereby following proper safety procedures.