For words that only have three letters, there is not any 4th letter. As an example with the word “incorporate”, there aren't any extra letters Later on and therefore needs to be the word "insert". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. http://mariobrpx591.cavandoragh.org/creating-your-copyright-safeguard-exactly-how-to-utilize-the-ian-coleman-bip39-generator-for-secure-budgets