O-4

In the US Armed Forces, servicemembers pay grades and ranking are set by codes. Ranks are numbered from lowest to highest. There are several classifications, E (Enlisted), WO (Warrant Officer), CWO (Chief Warrant Officer), and O (Officer). O-4 is therefore a mid grade officer rank, being above an O-3 and below an O-5. In the Army, this would be a Major and in the Navy this would be a Lieutenant Commander.