For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “add”, there aren't any extra letters afterwards and so have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've utilized “addi” for “addict”. By https://www.storeboard.com/blogs/beauty-and-fashion/opening-the-globe-of-bip39-a-comprehensive-guide-to-mnemonic-expressions-and-word-checklists/6026188