The HK4 is a pocket-size pistol of German origin. It was the first pistol to be produced by Heckler & Koch. The name HK4 was derived from being available in four calibers. It also was one of the first modular designs, being convertible to each of the four calibers.
The HK4 has some features of the Mauser HSc but is different in many aspects. The HK4 is a double action pistol that uses the straight blowback principle. Besides for the grip plates the pistol is fully made out of metal. An internal slide stop mechanism is fitted, which disengages by inserting a magazine. Fixed iron sights are used for aiming.
The HK4 is a semi-automatic pistol and was available in four calibers. Most were produced in .380 ACP and 7.65mm Browning. The other two options are .25 ACP and .22 Long Rifle. In .380 ACP the magazine holds 7 rounds, in all other models 8 rounds.