For words that only have 3 letters, there isn't any 4th letter. One example is with the word “add”, there aren't any additional letters Later on and so need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you might have utilized “addi” for “addict”. https://bookmark-nation.com/story19311911/fascination-about-invalid-bip39-phrase