For words that only have three letters, there isn't any 4th letter. As an example with the word “add”, there aren't any far more letters afterwards and thus must be the word "add". The word “addict” is inside the BIP39 wordlist however you would have made use of “addi” for https://pageoftoday.com/story4222843/getting-my-bip39-words-to-numbers-to-work