According to Wikipedia:
The HK 4 pocket pistol was first introduced by Heckler & Koch in 1967 making it the first pistol produced by the company. Mass production of this pistol started in 1968 and continued until 1984. There were approximately thirty-eight thousand pistols produced during this time. Serial numbers 10001 to 36550 were for commercial distribution. Twelve thousand pistols were produced for the German police and other government agencies in the .32 ACP (7.65×17mm Browning SR) caliber with the designation P11 and were serial numbered 40001 to 52400.
It is thought that the basic design of the HK 4 is based on that of the Mauser HSc. The HK 4 was designated “4” because the pistol had the ability to be modular in four calibers. 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 fourth caliber, .22 Long Rifle (.22 LR), 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. The three centerfire calibers that the pistol can use are: .25 ACP (6.35×16mmSR); .32 ACP (7.65×17mm Browning SR); .380 ACP (9×17mm Short). Most standalone pistols were sold in .380 ACP and .32 ACP. All four calibers came as a two-box set with the extra barrels, magazines and a screw driver/cleaning kit in accompanying box. Read full description in Wikipedia
The HK 4 is a straight-blowback, double-action to single-action pistol. 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 pistol automatically cocks the hammer after a round is fired. The safety incorporates a decocking mechanism that can safely decock the pistol with a round in the chamber. 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. When using center fire cartridges, there is a red marking on the top of the extractor that serves as a “chamber loaded” indicator. This feature has continued in current H&K pistols. The pistol incorporates a plastic buffer in the frame. The buffer, which is loosely installed in the frame, can be lost during cleaning or can wear out and needs to be replaced periodically.
Country : Germany
Gun Name : Heckler & Koch HK4
Manufacturer / Gun Brand : Heckler & Koch GmbH
Calibre : .380 ACP, 7.65mm (.32ACP) 6.35mm (.25 ACP),.22 Long Rifle
Magazine Capacity : 7 (.380 ACP ) or 8
Action : Blowback / Double
Total Length : 157mm / 6.18in
Barrel Length : 85mm / 3.35in
Weight Unloaded : 480g / 16.9oz
Image Credit: Courtesy Pistol Heckler und Koch HK4 http://www.adamsguns.com/