For words that only have three letters, there is no 4th letter. As an example with the word “include”, there isn't any far more letters afterwards and thus has to be the word "include". The word “addict” is within the BIP39 wordlist but you would have utilized “addi” for “addict”. https://dailybookmarkhit.com/story19012198/the-basic-principles-of-bip39-passphrase-generator