Which military rank corresponds to the O-4 designation?

The rank that corresponds to the O-4 designation is Captain. In the context of military rank classification, O-4 is used primarily in the United States Armed Forces to identify the rank of Captain in the Army, Air Force, and Marine Corps. In the Navy, however, O-4 is the rank of Lieutenant Commander.

Captains (O-4) usually command companies, squadrons, or equivalent levels in their respective branches and are responsible for leading groups of enlisted personnel and junior officers. Their duties can include strategic planning and operations management, which align with the responsibilities expected at an O-4 rank.

While Lieutenant Commander is indeed an O-4 rank in the Navy, it does not apply to all branches. Therefore, in a broader context, Captain is more universally recognized across multiple military services as the rank corresponding to O-4.