A larger engine, the Lycoming O-290B with 125 hp, would be installed in the aircraft, making it the S-4A.

Seibel also replaced that aircraft's original wheeled, tricycle undercarriage with landing skids.

A lower deck supported the control panel, pilot's seat, wheeled, tricycle landing gear, and a small passenger/cargo area accessible from the rear, and an upper deck carried the engine, the fuel and oil tanks, and supported the transmission and rotor assembly.

In early 1951, the U.S. Army ordered two examples for operational and engineering evaluation in the observation, utility, and aeromedical evacuation roles.

Despite the simplicity of the S-4, the Army determined that it did not provide a sufficient payload capability and the aircraft were dropped from the inventory and returned to Seibel in 1952.