For words that only have three letters, there's no 4th letter. For instance with the word “add”, there aren't any much more letters afterwards and so have to be the word "include". The word “addict” is while in the BIP39 wordlist however, you would have applied “addi” for “addict”. Mnemonic https://eternalbookmarks.com/story18651427/fascination-about-invalid-bip39-phrase