For words that have only 3 letters, there's no 4th letter. As an example with the word “include”, there are no more letters afterwards and therefore need to be the word "add". The word “addict” is while in the BIP39 wordlist however you would have utilized “addi” for “addict”. Using https://mysterybookmarks.com/story19437314/the-basic-principles-of-bip39-passphrase-generator