The X4 was an Italian submachine gun designed and manufactured by the Italian firearms manufacturer Fabrica Nazionale d’Armie Brescia throughout the mid-1950's.
The X4 was a 9mm submachine gun designed in 1954. A very basic but effective weapon, it was very cheap to produce and the original manufacturing run consisted of about 100,000 units. A single X4 submachine gun is estimated to have cost about $15 to manufacture. Sales were low, however. Contemporaries commented that the X4 was actually a very handy and efficient weapon, but there were no military or security forces willing to place any orders. In 1955, a shortened model known as the X5 was produced, which was probably one of the shortest submachine guns in existence at the time of its production, but this did not help sales.
Both the X4 and the X5 were constructed almost entirely from stamped sheet metal. The bolt was cylindrical with a fixed firing pin and the cocking handle was attached to a sleeve that wrapped around the entire bolt. The magazine housing was especially deep. Only fully-automatic fire was possible. The X4 utilized a retractable wire stock, but the X5 did not.