The Ruhrstahl Ru 344 X-4 or Ruhrstahl-Kramer RK 344 was a wire-guided air-to-air missile designed by Germany during World War II. The X-4 did not see operational service and thus was not proven in combat but inspired considerable post-war work around the world, and was the basis for the development of several ground-launched anti-tank missiles.
The first flight test occurred on August 11, 1944, using a Focke-Wulf Fw 190 for the launch platform. Subsequent tests used the Junkers Ju 88 and Messerschmitt Me 262, although they were not launched from the latter. The X-4 had originally been intended for use by single-seat fighters (including the Me 262 and possibly the Dornier Do 335),but the problems in guiding both the missile and the aircraft at the same time proved unworkable. Instead, the X-4 was re-directed to multi-seat aircraft like the Ju 88, while the unguided R4M rocket was to be used in single-seaters.