What is "four-letter words"

Usage examples of "four-letter words".

If she couldn't say out loud all those four-letter words, how was she supposed to relay the message from the potty-mouthed man in Las Vegas?

Its walls were fine oak but on one panel there was a concentration of four-letter words that the old man covered immediately, stepping square in front of them without appearing to notice and then staring upward.

I don't mean, Why do men have four-letter words tattooed on their hands?

She had leaped up and began to pound on her desk and rave and rant in four-letter words that even I had never heard.

Hammet's vocabulary had mellowed greatly since he'd come down from the cabin, and I certainly didn't want to remind him of his innate talent for four-letter words and quaint colloquialisms, most of which concerned farm animals and improbable sexual activities.