For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “add”, there aren't any additional letters Later on and so must be the word "insert". The word “addict” is within the BIP39 wordlist however you might have used “addi” for “addict”. https://privatebookmark.com/story19015624/a-secret-weapon-for-bip39-words-to-numbers