As HK’s first pistol, I always thought the HK4 was a neat concept: a compact double-action blowback pistol that could be adapted to fire multiple calibers (in this case: .380, .32acp, .25acp and .22LR). Just replace the barrel/spring assembly and use the correct magazine and Voila! Shooty goodness with 4 common chamberings. Based on the earlier Mauser HSc pistol design, the HK4 used a cast alloy frame and a slide made out of welded steel stampings, a cost-saving technique which Sig later adopted.
The concept didn’t really take off. The gun was available in 4 calibers, but each caliber required a different magazine. Using the .22LR barrel required moving/reversing the extractor faceplate/firing pin assembly, and a special screwdriver (with cleaning kit in the handle!) was included with the kit to enable this. The design incorporated a plastic buffer in the frame which could fall out; in any case the buffer required periodic replacement. (Crapgame just LOVES this design feature in a gun). Early examples required different extractors that had to be swapped out when changing calibers; later guns had a universal extrator which eliminated that issue.
HK made just 38,000 HK4s between 1968 and 1984, which includes 12,000 in .32acp for the German police (renamed P11 for German issue). There were some French HK4′s made by MAS/Manurhin to fulfill needs in Berlin where German-made guns were forbidden. H&R imported some as co-branded HK4s (primarily as .380-only pistols, and .380/.22LR sets) in 1971 to celebrate their 100th year in operation; a small number of the 4-caliber sets were imported as well.
I have seen a few HK4s at gun stores and gun shows over the years. I have been tempted, but most of the examples I found were orphans, with only 1 extra barrel (sometimes none) and no special screwdriver. The few complete 4-barrel kits I have seen were way overpriced. I have never seen an HK4 with more than one magazine per caliber. And that buffer-replacement issue is something I don’t want to deal with.
Caliber conversion is a tempting feature. Nowadays, .22LR conversion kits are available for AR15s, Mini-14s and 1911s. Glocks in .357Sig and .40S&W can be easily adapted to fire the other caliber. But true multi-caliber versatility is a feature with high engineering costs and only a niche market to sell to.