What does "4 down/up" refer to in aircraft components?

The term "4 down/up" specifically refers to the position of the IFR (Instrument Flight Rules) probe on an aircraft. When the probe is described as "4 down," it indicates that the IFR probe is extended or deployed, allowing the aircraft to engage in refueling operations while in flight. Conversely, "4 up" would reference the probe being retracted, indicating that it is stowed away when not in use.

This terminology is crucial for pilots and ground crew to communicate effectively regarding the status of the IFR probe, as it plays an essential role in aerial refueling and overall aircraft functionality. Understanding the correct position of this component ensures that operations are performed safely and efficiently, greatly aiding in mission readiness.