For words that only have three letters, there is not any 4th letter. As an example with the word “include”, there aren't any additional letters afterwards and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would've applied “addi” for “addict”. https://trackbookmark.com/story21101522/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting