The phrase 'four-letter word' is a polite and inoffensive way to refer to swear / dirty words. From this, phrases like 'moving is a four-letter word' or, 'coming from you, "friend" is a four-letter word' have arisen to describe the distasteful.

Of George Carlin's Seven Dirty Words, 5 (fully 71.4%) of them are four letters long. Of his 2443 Filthy Words and Phrases, the most frequently encountered word/phrase length is in fact four letters (231 words or phrases, (roughly 9.5%)). Next is six letters (213), then seven (208). Note that the version of George Carlin's 2443 Filthy Words and Phrases used, transcribed by Patrick Arnold, consisted of only 2439 entries.

Finally, of the 27 very mild / mild / moderate / strong naughty words in the BBFC's Categories Of Swear Words, 10 (37%) of them are four letters long.

To compare, only 2236 (less than 5%) of linux.words' 45424 words are four letters in length. Words of five, six, seven, and eight characters weigh in at 4176 (9.9%), 6177 (13.6%), 7375 (16.2%), and 7077 (%15.6) respectively.1

Sources:

EFF's "Legal Cases - FCC v Pacifica (George Carlin '7 Dirty Words' Case)" Archive
http://www.eff.org/Censorship/Radio_and_tv/FCC_v_Pacifica/
George Carlin's 2443 Filthy Words and Phrases (as transcribed by Patrick Arnold):
http://www.primenet.com/~btrosko/pages/dirty-words.html

1 Percentages are rounded to one significant digit.

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