The Rurhstahl X4 was a liquid fueled, wire guided, airtoair missile in development by Germany in WWII. While too late to have any impact, it did provide research into the future development of wire guided missile technology.

By late 1944 about 1,300 missile assemblies had been built, but the factory making the motors had been heavily bombed leaving few motors to be used. Ironically, the Allied bomber formations that these missiles were intended for destroyed the capability of these missiles from reaching Luftwaffe units.