For words that only have three letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any additional letters afterwards and so have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've applied “addi” for “addict”. The https://bookmarkmargin.com/story18808645/fascination-about-invalid-bip39-phrase