For words that only have three letters, there is not any 4th letter. For instance with the word “include”, there aren't any additional letters Later on and thus have to be the word "increase". The word “addict” is inside the BIP39 wordlist however you might have made use of “addi” http://remingtonjszi611.almoheet-travel.com/from-words-to-wide-range-how-to-efficiently-use-bip39-passphrases-for-your-copyright-wallets