For words that only have three letters, there's no 4th letter. As an example with the word “incorporate”, there are no additional letters afterwards and so should be the word "add". The word “addict” is from the BIP39 wordlist however you might have employed “addi” for “addict”. In this way, https://bookmark-share.com/story18905157/the-basic-principles-of-bip39-passphrase-generator