For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there aren't any more letters afterwards and thus have to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would've made use of “addi” for “addict”. Art https://iowa-bookmarks.com/story15401346/a-review-of-bip39-passphrase