Which of the following is a function of Mode 4?

Mode 4 is specifically designed to provide secure identification for military aircraft to enhance friendly force identification and to prevent accidental engagement by friendly fire. This mode utilizes encrypted communication to ensure that only authorized military aircraft can be identified positively, thereby increasing operational security during military operations.

The other options do not align with the functions of Mode 4. Identifying commercial aircraft pertains more to civilian aviation protocols, while communicating with ground control is a function associated with air traffic management rather than military identification systems. Measuring weather conditions is entirely outside the purview of Mode 4's capabilities, as it is focused strictly on aircraft identification rather than environmental data gathering.