For words that only have three letters, there isn't any 4th letter. As an example with the word “add”, there isn't any extra letters afterwards and thus needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you might have used “addi” for “addict”. https://madesocials.com/story4251898/little-known-facts-about-bip39-words