The R-4 was the US military designation of the VS-316A, essentially an improved VS-300 with a fully enclosed, fabric-covered structure (the last metre or so of the tail skeleton was left uncovered), slightly larger main rotor diameter, improved engine and without the small, nose-mounted stabilising wheel.
Following promising performance by a prototype, United Aircraft (Vought-Sikorsky Division) accepted a contract in 1941 to develop it further into a twin seater. On 14 Jan 1942, the first (XR-4) Hoverfly was flown. After a historic flight of over 750 miles (in several stages between 13 and 17 May) and acceptance in May 1942, the US ordered 30 machines (YR-4) in 1943.
Bristow Commemoration of Helicopter Landing, 26-Sep-16 : Seventy years ago, on September 24, 1946, pioneering helicopter pilot, Lieutenant Alan Bristow Royal Navy, became the first Briton to land a helicopter on the deck of a naval frigate at sea