For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there isn't any a lot more letters afterwards and thus has to be the word "insert". The word “addict” is from the BIP39 wordlist however you might have made use https://freebookmarkpost.com/story18683145/5-simple-techniques-for-bip39-word-list-pdf