For words that have only three letters, there is no 4th letter. By way of example with the word “add”, there isn't any extra letters Later on and therefore should be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. Standard https://web.archive.org/web/20241123074420/https://bip39-phrase.com/