For words that only have 3 letters, there's no 4th letter. One example is with the word “incorporate”, there isn't any far more letters afterwards and therefore need to be the word "insert". The word “addict” is during the BIP39 wordlist however , you would have employed “addi” for “addict”. https://fellowfavorite.com/story19960057/a-review-of-bip39-passphrase