For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there are no far more letters afterwards and thus have to be the word "add". The word “addict” is inside the BIP39 wordlist but you might have utilized “addi” for “addict”. https://maximusbookmarks.com/story19611941/fascination-about-bip39-english-words