For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there aren't any additional letters afterwards and therefore has to be the word "increase". The word “addict” is from the BIP39 wordlist however you would have utilised “addi” for “addict”. Mnemonic https://total-bookmark.com/story18902770/5-essential-elements-for-bip39-explained