For words that only have three letters, there isn't a 4th letter. By way of example with the word “include”, there isn't any additional letters afterwards and therefore must be the word "include". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. BIP39 and https://tinybookmarks.com/story18938412/not-known-factual-statements-about-bip39-explained