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.

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 BMW engines and production lines, a blow from which the program never recovered. This artifact was a gift of the Thiokol Chemical Corporation, Reactions Motors Division, which had acquired it from the U.S. military for post-World War II testing.