For words that only have 3 letters, there's no 4th letter. Such as with the word “add”, there won't be any more letters Later on and therefore have to be the word "include". The word “addict” is while in the BIP39 wordlist however you would have employed “addi” for “addict”. https://escatter11.fullerton.edu/nfs/show_user.php?userid=7804586