The Seibel S-4 was a two-bladed, single-engine helicopter built by Seibel Helicopter. Designed by Charles Seibel, the S-4 was evaluated by the United States Army under the designation YH-24 Sky Hawk, but would be rejected for service. The S-4B would serve as the basis for the design of the Cessna CH-1 Skyhook, the only helicopter Cessna ever produced.