For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there isn't any much more letters Later on and so need to be the word "increase". The word “addict” is in the BIP39 wordlist however you might have made use of https://e-bookmarks.com/story4452986/5-simple-techniques-for-bip39-word-list-pdf