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 thus needs to be the word "increase". The word “addict” is in the BIP39 wordlist however you would have utilised “addi” for “addict”. An https://www.slideserve.com/lewarthbfp/bip39-word-list-explained-your-essential-source-for-creating-secure-purses-in-t