The BOR-4 flight vehicle is a scaled (1:2) prototype of the Soviet Spiral VTHL (vertical takeoff, horizontal landing) spaceplane. An uncrewed, subscale spacecraft, its purpose was to test the heatshield tiles and reinforced carbon-carbon for the Buran space shuttle, then under development.

Several of them were built and flown between 1982 and 1984 from the Kapustin Yar launch site at speeds of up to Mach 25. After reentry, they were designed to parachute to an ocean splashdown for recovery by the Soviet Navy. The testing was nearly identical to that carried out by the US Air Force ASSET program in the 1960s, which tested the heatshield design for the X-20 Dyna-Soar. On June 3, 1982 a Royal Australian Air Force P-3 Orion reconnaissance aircraft captured the first Western images of the craft as it was recovered by a Soviet ship near the Cocos Islands.