For words that have only three letters, there is not any 4th letter. For instance with the word “incorporate”, there aren't any more letters afterwards and so has to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however , you might have made use of https://sergiorgzi464.weebly.com/blog/from-words-to-wealth-exactly-how-to-properly-use-bip39-passphrases-for-your-copyright-budgets