Which of the following ranks is classified as O-4?

The rank classified as O-4 is the Lieutenant Commander. This rank is part of the officer ranks in the military and is specifically designated for officers in the pay grade of O-4. It reflects a level of experience and responsibility, typically associated with leadership roles and some decision-making authority within various operations and units.

Lieutenant Commanders may serve in a variety of roles across different branches of the military and are often found in leadership positions, overseeing junior officers and enlisted personnel. This rank comes after Lieutenant (O-3) and before Commander (O-5), which emphasizes the ongoing progress in rank and the development of responsibilities that an officer accumulates over time.