For words that have only three letters, there is absolutely no 4th letter. For instance with the word “insert”, there aren't any a lot more letters Later on and therefore must be the word "insert". The word “addict” is inside the BIP39 wordlist however, you would've used “addi” for “addict”. https://thegreatbookmark.com/story18845204/a-review-of-bip39-passphrase