For words that have only 3 letters, there is no 4th letter. One example is with the word “include”, there isn't any far more letters Later on and thus needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have made use of https://www.tumblr.com/generalllamacolor/772952501127380992/opening-the-globe-of-bip39-a-comprehensive-guide