For words that only have three letters, there's no 4th letter. By way of example with the word “include”, there isn't any much more letters Later on and therefore needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would've applied “addi” for “addict”. https://isocialfans.com/story4284147/a-review-of-bip39-phrase-list