For words that have only 3 letters, there isn't any 4th letter. For instance with the word “add”, there won't be any more letters Later on and thus should be the word "add". The word “addict” is during the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://www.animenewsnetwork.com/bbs/phpBB2/profile.php?mode=viewprofile&u=1025088