For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters Later on and thus needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would've employed “addi” for “addict”. A simple https://tbookmark.com/story18698999/the-basic-principles-of-bip39-passphrase-generator