For words that only have 3 letters, there's no 4th letter. One example is with the word “include”, there are no far more letters Later on and thus needs to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have employed “addi” for “addict”. https://socialwebconsult.com/story4249065/a-review-of-bip39-phrase-list