Model Variants (Including Prototypes)
XR-4 - Single Prototype Model; fitted with R-500-3 series engine of 165 horsepower.
YR-4A - Developmental model with R-550-1 engine of 180 horsepower; larger-diameter rotor for improved lift; 3 examples completed.
YR-4B - Development model for formal US Army evaluations; 27 examples produced.
R-4B - Production Designation of R-4 series; fitted with R-550-3 series engines of 200 horsepower; 100 examples produced.
XR-4C - Prototype model with R-550-1 engine of 180 horsepower; revised rotor.
HNS-1 - US Navy designation of three YR-4B and twenty-two R-4B models.
"Hoverfly I" - British military designation of R-4 production model; 52 examples delivered.
S-54 - R-4B test platform with observer's position added.