For words that have only three letters, there's no 4th letter. As an example with the word “add”, there aren't any much more letters Later on and thus needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however you would have made use of “addi” https://peakbookmarks.com/story18937431/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting