O4 means Officer grade: Captain or Colonel in Military
Similar to O4
48 FW - 48th Fighter Wing 4ID - 4th Infantry Division 4WW - The Fourth World War 503 RCT - 503rd Regimental Combat Team 517 PRCT - 517th Parachute Regimental Combat Team 596 SSC - 596th Signal Support Company 5CCGP - Fifth Combat Communications Group