The S-4 was the brainchild of Charles Seibel. It was an attempt to break into the lucrative rotorcraft market that the United States Army was creating. The idea was simplicity, a welded steel tool box frame, a double blade rotor a tapered ail boom and a twin bladed anti torque fan at the end.
The first iteration of the S-4 flew in 1949 and performed nicely. Later the cockpit was widened to include two people and it was driven by a 125 horsepower Avco-Lycoming engine. The Army lied it but felt it lacked payload capability and dropped it after the first two prototypes.
The S-3, prototype to the S-4
Charles Seibel Explaining The Helicopter He Created To Kansas Governor Frank Carlson.