For words that have only three letters, there isn't a 4th letter. One example is with the word “add”, there won't be any a lot more letters afterwards and thus needs to be the word "include". The word “addict” is within the BIP39 wordlist however, you would've made use of https://letusbookmark.com/story20675288/5-simple-techniques-for-bip39-word-list-pdf