Lists of English words four letters in length can be found at websites such as Word Find, Your Dictionary and Scrabble Wizard. Note that "four-letter word" is also an English idiom for curse words; the website NoSwearing lists English four-letter words in this slang sense.
Several dictionary websites, including Your Dictionary, have subsections that sort words according to length. The Word Finder section of Your Dictionary lists four-letter words as an aid for word games. Similarly, Word Wizard includes a list of four-letter words for Scrabble aficionados. Their list features the top-scoring four-letter words in that word game. Word Find's list four-letter word list also includes a Scrabble word generator.
All three of these websites list four-letter words without regard to their vulgarity. Four-letter words, in the sense of taboo words, are listed at NoSwearing, a website that focuses on English curses and swears.