(NSFW) -- JIC. Why is so much of English language profanity four letters long?

I don’t know if this is Not Safe for Work (NSFW), but given the subject matter, Just in Case (JIC) the answers go there, I’ll put the warning up. Sure, there are the exceptions like the “B” words and the three-letter word for the non-profane anus—which, remarkably—is a corruption of the British word, arse (four letters). But so many swear words are four letter words that four letter word has become a euphemism for cursing. For the linguists among us, is this just a coincidence, or is there some subtle bias about four letters being profane that crept in from Proto-Indo-European roots, old German, Latin or what?