The mysteries of English orthography result in many words in that language appearing in writing with four letters. This expression does not refer to all such words.

Some few of those words, scorned in polite society but redolent with earthy Anglo-Saxon pungency, have become known simply as four-letter words. Common popular or slang terms for excretory functions, sexual activity or genital body-parts (for example) thus have their own artificial and euphemistic linguistic category.

Common four-letter words (in this sense) include: cunt, damn, fuck, and shit. All of these words are considered vulgar or offensive. See profanity.

A similar tradition occurs in Dutch although it's using only three letters and are in fact three-letter-words: (kut, pik, lul, gvd)

Another interesting meaning of four-letter word (in Greek, tetragrammaton) is "the name of God", that is, יהוה (commonly transliterated as "YHVH"), which was supposed never to be spoken aloud, now known as Yahweh or Jehovah.

Quotation

Good authors too who once knew better words
Now only use four-letter words
Writing prose.
Anything goes.
Cole Porter, "Anything Goes"