For words that only have three letters, there isn't any 4th letter. One example is with the word “add”, there won't be any additional letters Later on and thus must be the word "incorporate". The word “addict” is during the BIP39 wordlist but you would've utilized “addi” for “addict”. Try https://checkbookmarks.com/story4575017/a-review-of-bip39-passphrase