The German X-4 was a small World War II air-to-air missile that could be fired at heavily armed Allied bombers from a distance. To prevent jamming, guidance was trasmitted by wires running between the missile and launch aircraft. Slated for use on the Me 262 jet fighter, the X-4 could also have been fired from such piston-engine aircraft as the Ju 88, Ju 388, and Fw 190, all of which launched test missiles beginning in August 1944.
A BMW 109-548 liquid-fuel rocket engine powered the missile. Ruhrstahl produced 1,000 X-4 airframes in late 1944, but an Allied air raid destroyed the engines and production lines, a blow from which the program never recovered. Nothing is known about the origins of the Smithsonian's artifact except that it came to the NASM in 1971 as part of a U.S. Navy gift of early German and American experimental missiles and glide bombs.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.