The German X-4 was a small air-to-air missile of World War II that could be fired at heavily armed Allied bombers from a distance. To prevent jamming, guidance was by wires running between the missile and launch aircraft, rather than by radio. 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.

Ruhrstahl produced 1,000 X-4 airframes in late 1944, but an Allied air raid destroyed the BMW engines and production lines, a blow from which the program never recovered. The Smithsonian obtained the missile body from the U.S. Navy in 1948. The wings and other components come from another Navy transfer in 1971. In 2017, the Deutsches Technikmuseum assembled these components into a complete missile under a restoration-loan contract.