For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there won't be any additional letters afterwards and so should be the word "insert". The word “addict” is during the BIP39 wordlist however you would've used “addi” for “addict”. https://social-galaxy.com/story4251265/a-review-of-bip39-passphrase