For words that only have three letters, there isn't a 4th letter. As an example with the word “add”, there isn't any extra letters afterwards and so need to be the word "include". The word “addict” is within the BIP39 wordlist however, you would have utilised “addi” for “addict”. This https://socialrator.com/story9402081/the-2-minute-rule-for-bip39-all-words