For words that only have 3 letters, there isn't a 4th letter. One example is with the word “incorporate”, there aren't any much more letters Later on and so have to be the word "add". The word “addict” is inside the BIP39 wordlist however , you would've made use of https://naturalbookmarks.com/story18918199/not-known-factual-statements-about-bip39-explained