For words that have only three letters, there isn't a 4th letter. One example is with the word “add”, there aren't any extra letters afterwards and so must be the word "increase". The word “addict” is in the BIP39 wordlist however you would've made use of “addi” for “addict”. gravity https://sociallweb.com/story4283477/5-essential-elements-for-bip39-explained