For words that only have three letters, there isn't a 4th letter. For example with the word “add”, there are no a lot more letters afterwards and so have to be the word "add". The word “addict” is during the BIP39 wordlist however, you would have employed “addi” for “addict”. https://ok-social.com/story4512027/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool