For words that only have 3 letters, there is not any 4th letter. For instance with the word “add”, there isn't any far more letters afterwards and so needs to be the word "insert". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. In https://socialmediainuk.com/story20491814/a-review-of-bip39-passphrase