For words that have only 3 letters, there isn't any 4th letter. Such as with the word “incorporate”, there aren't any extra letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however you would've made use of “addi” for “addict”. In https://bookmarkinglog.com/story18802673/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition