Heckler & Koch got its start making rifles for military and police use, but the demand for semi-automatic pistols grew in the 1960s, leading to the HK4.
The one pictured is a .32; I also have a .22 conversion for it...I have another one in .32 which is much better condition, and doesn’t get out much.
One thing to slightly correct in the article—the .380’s are relatively uncommon; the .380 was a bit much for a pistol designed as a .32, so it tends to destroy the rubber recoil buffer quickly, and then beats up the frame.
Most HK4’s are .32, with .22 conversions being the second most common; .380’s were next, and .25’s are the least common...usually only found with the complete, 4-caliber conversions.