EVery 8-year-old knows the list of disreputable Anglo-Saxon epithets, most of which, except for derivatives and compound formations, coincidentally are spelled with four letters, hence the term “four-letter words.” But why? Is there some metalinguistic reason beyond being related to their monosyllabity? Who, if anyone, settled on four letters, and why?
I don’t think there’s any particular reason why so many curse words consist of four letterrs. Many more perfectly wonderful and acceptable words are also spelled with four letters than curse words, in fact. My guess is they collapsed to that state under the staggering pressure of sheer coincidence.
I ran eight of the most common ones through Merriam-Webster and found that
[ul][li]they had been introduced gradually between the C12[sup]th[/sup] and C19[sup]th[/sup][/li][li]six of the eight were derived from words that previously been spelt with other than four letters[/ul][/li]So unless someone can trace some evidence for a conspiracy among linguists over the last eight hundred or so years the obvious conclusion must be coincidence, and the term “four letter word” followed from somebody noticing it.
Also, the classic four-letter words derive from Anglo-Saxon through Germanic roots. These languages tended to have short words, few syllables, and sharp sounds (“ck”) as opposed to words of latinate derivation, which are multi-syllabic and contain a higher ratio of vowels. (Yes, I know how oversimplified this is. 
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Even so, not all the “classic” four letter words are derived from Germanic roots. Piss and probably crap came to us via Old French for example. It’s still far fetched to imagine that spelling so many of them with four letters is deliberate.
It’s not just the Germanic roots, it’s how the words are used and when. Think of some short words you know of… “House”, “Dog”, “Man”, etc. Now think of some of the long words you know: “Deoxyribonucleic”, “Brachycardia”, “Multisyllabic”. Now, which sort of words do you use often, in day to day life? Most commonly used words are short, and most technical terms are long. So vulgarisms tend to be short. As to why four letters, I imagine that any shorter, and there’s a shortage of good words to go around. But once you get up to four letters, you’ve got room for a lot of variety.
I doubt if it’s significant, but there’s a professor of Computer Science somewhere who used to perform the “monkeys on a typewriter” experiment, letting random numbers determine which letters fall next. At first he used truly random probabilities (letting a space be a “letter”), and understandably got gibberish. Then he used the relative probabilities of the usages of the letters to determine whoch ones should occur, and it didn’t look so random. Then he tried “second order” probabilities – taking pairs of letters and assigning probabilities to their occurrence, then using those to determine the likelihood that you computer monkeys would type that combination. Things started looking like real words. He did better with “third order” monkeys, using the probabilities of sets of three letters (in order).
When he tried “fourth order” monkeys he found something interesting – a lot of dirty words showed up. (“Longue ass kisses” in one draft) He wondered if the prevalence of risque four-letter words had something to do with their likelihood of showing up in such quasi-random generators.
His monkeys never did produce any Shakespeare, by the way, even when he used Shakespearian text to determine the probabilities. “Not enough randomness in the quasi-random number generator” was his pronouncement.