To reassemble the pistol, insert the barrel back in the slide. The barrel can now be removed from the slide. A notch inside the front of the trigger guard at the frame must slide down and be held in position while moving the slide slightly forward and then up to detach it from the frame. The pistol is disassembled by removing the magazine, cocking the hammer and putting the safety in the 'safe' position. The buffer, which is loosely installed in the frame, can be lost during cleaning or can wear out and needs to be replaced periodically. The pistol incorporates a plastic buffer in the frame. This feature has continued in current H&K pistols. When using center fire cartridges, there is a red marking on the top of the extractor that serves as a 'chamber loaded' indicator. The hammer is decocked by putting the safety in the 'safe' position and pulling the trigger, which allows the hammer to fall without striking the firing pin. The safety incorporates a decocking mechanism that can safely decock the pistol with a round in the chamber. The pistol automatically cocks the hammer after a round is fired. With a round in the chamber, a bullet may be fired by either pulling the trigger or by cocking the hammer and then pulling the trigger.

The HK 4 is a straight-blowback, double-action to single-action pistol. All four calibers came as a two-box set with the extra barrels, magazines and a screw driver/cleaning kit in accompanying box. The three centerfire calibers that the pistol can use are. 22 Long Rifle (.22LR), requires the extractor faceplate and firing pin be moved to conform to the proper configuration for rimfire operation in addition to changing the barrel and magazine. This means the frame and slide could accommodate three (3) different centerfire calibers by simply replacing the barrel/return spring assembly and inserting the proper magazine for the caliber of choice. The HK 4 was designated '4' because the pistol had the ability to be modular in four calibers.

It is thought that the basic design of the HK 4 is based on that of the Mauser HSc. 32 ACP (7.65x17mm Browning SR) caliber with the designation P11 and were serial numbered 40001 to 52400. Twelve thousand pistols were produced for the German police and other government agencies in the. Serial numbers 10001 to 36550 were for commercial distribution. There were approximately thirty-eight thousand pistols produced during this time.

Mass production of this pistol started in 1968 and continued until 1984. The HK 4 pocket pistol was first introduced by Heckler & Koch in 1967 making it the first pistol produced by the company. Fixed, front post and rear notch, sight radius 121 mmm

Mauser hsc firing pin removal