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.

Work on the X-4 began in June 1943, by Dr Max Kramer at Ruhrstahl AG. The idea was to build a missile with enough range to allow it to be fired from outside the range of the bombers' guns.
The X-4 met these specifications and more. Its BMW 109-448 rocket motor accelerated the missile to over 1,150 km/h (710 mph) and kept the X-4 at this speed during its "cruise", between 1.5 and 4 km (0.9 and 2.5 mi).
Signals to operate control surfaces on the tail were sent via two wires (a method chosen to avoid radio jamming), which unwound from bobbins with a total of about 5.5 km (3.4 mi; 3.0 nmi) of wire.
The wires were controlled by a joystick in the cockpit.
The warhead consisted of a 20 kg (44 lb) fragmentation device that had a lethal radius of about 8 m (26 ft).
The missile mounted a proximity fuze known as a Kranich (Crane), using an acoustical system.

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 was designed to be easily assembled by unskilled labour and airframe production began in early 1945. Production was hampered by Allied bombing of the BMW rocket engine factory at Stargard, though as many as 1,000 X-4s may have been completed, the missile was never officially delivered to the Luftwaffe.
The X-4 did not see operational service but inspired considerable post-war work around the world, and was the basis for the development of several ground-launched anti-tank missiles.
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