For words that only have 3 letters, there's no 4th letter. For example with the word “add”, there isn't any additional letters afterwards and thus need to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would have utilised “addi” for “addict”. An easy backup https://prbookmarkingwebsites.com/story21860773/the-basic-principles-of-bip39-passphrase-generator