During the manufacture of the SU-2’s and SU-3’s, Vought had produced an improved model designated the O3U-6, incorporating an improved P&W Wasp engine.  Although the Navy approved of the design the initial order was placed for 20, which were designated SU-4’s since they were delivered to Marines to replace their SU-1’s aboard the USS Lexington and USS Saratoga.

An updated O3U-6, the main changes being the use of the latest P&W R-1690-42 engine.  These were the latest Corsair II’s ordered by the Navy, and utilized the same SU-4 designation as the prior model Corsair aircraft.  In 1941, the Naval Aircraft Factory converted some of these aircraft into target drones and radio-controlled drones for flight testing under hazardous conditions.  Some flew as late as 1942 at various Navy training facilities.