For words that have only three letters, there isn't any 4th letter. One example is with the word “add”, there aren't any extra letters afterwards and therefore must be the word "include". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. bip39-phrase.com https://admiralbookmarks.com/story18828595/the-invalid-bip39-phrase-diaries