For words that have only three letters, there isn't any 4th letter. For example with the word “add”, there are no extra letters Later on and so needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have employed “addi” for “addict”. https://highkeysocial.com/story4325692/the-ultimate-guide-to-bip39-seed-phrase-github