What does the M4 alarm indicator of the IFF signify when it lights up?

The M4 alarm indicator of the Identification Friend or Foe (IFF) system is designed to inform operators about the status of the mode 4 codes. When this alarm lights up, it specifically indicates a fault or that the mode 4 codes are not loaded. This is critical because mode 4 is a secure system used to differentiate between friendly and potentially hostile aircraft.

If the M4 alarm is on, it signals that there is a significant issue that needs attention, as the IFF system relies heavily on the presence of these codes for operational effectiveness. Understanding this is crucial for personnel involved in air defense operations, as it impacts their ability to make informed decisions in potentially volatile situations. In contrast, the other options imply states of functionality or readiness that do not accurately reflect the condition associated with an illuminated M4 indicator; they do not acknowledge the critical nature of ensuring mode 4 codes are properly functioning to avoid misidentification during air engagements.