Which situation falls under the Level Four threat category?

A situation that involves an attempted breach of the flight deck falls under the Level Four threat category because it represents a severe and immediate threat to the aircraft and its occupants. Level Four is characterized by high-risk incidents that pose a direct danger to the safety of the airplane. An attempted breach of the flight deck could lead to catastrophic outcomes, such as the aircraft being taken over or control being compromised. This situation typically involves hostile intent and immediate action is required to mitigate the danger, reflecting the heightened level of risk associated with such an event.

In contrast, a credible hijacking threat is serious but may not indicate an immediate action is being taken at that moment, fitting more into a different threat level. A passenger dispute, while it may escalate, generally does not pose a critical risk to the flight's safety and is more manageable in nature. Likewise, a routine medical emergency, while requiring attention, is not related to a security threat and is thus not categorized under this high-risk classification.