For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “include”, there are no extra letters afterwards and so have to be the word "include". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. The https://bookmarkspiral.com/story18862019/not-known-factual-statements-about-bip39-explained