For words that only have three letters, there is not any 4th letter. By way of example with the word “incorporate”, there are no additional letters afterwards and so have to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have made use of https://madbookmarks.com/story18798326/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase