For words that only have three letters, there isn't a 4th letter. By way of example with the word “add”, there isn't any additional letters afterwards and so must be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://pageoftoday.com/story4280615/the-basic-principles-of-bip39-passphrase-generator