For words that have only 3 letters, there is no 4th letter. One example is with the word “include”, there are no extra letters Later on and so have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've used “addi” for “addict”. A https://7bookmarks.com/story18867806/the-invalid-bip39-phrase-diaries