For words that have only three letters, there isn't any 4th letter. One example is with the word “include”, there aren't any extra letters afterwards and so have to be the word "include". The word “addict” is inside the BIP39 wordlist however you would've employed “addi” for “addict”. In this https://expressbookmark.com/story18844108/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator