The H&K Model 4 was named for the fact that it was offered in four different calibers – .22LR, .25 ACP, .32 ACP, and .380 ACP. The gun came with a complete set of spare barrels and magazines to allow conversion between all of them, and interesting feature not offered by any other pistols like it at the time. The design was by Alex Seidel, one of the founding engineers of H&K. He had familiarity with the Mauser HSc from his time working at Mauser, and it was the rough basis fo the HK4.
The changes between centerfire calibers required nothing more than swapping barrels (and recoil springs, which were pinned to the barrels) and magazines. Differing spring strength for each caliber were enough to make the gun both safe and reliable in the different chamberings. The convert too .22LR rimfire, it was also necessary to unscrew the removable breech face and flip it around. The breech face had two firing pin holes (one centerfire and one rimfire), and the firing pin could pivot enough house either one. When the breech face was removed, the firing pin could be positioned for whichever setting was desired.
The HK4 was interesting and reasonably successful, but never able to really compete with guns like the Walther PP and PPK. During a 16-year production run from 1968 until 1984 a total of 38,200 were made, including 12,400 for the German customs police.