For words that have only 3 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 "add". The word “addict” is from the BIP39 wordlist but you might have used “addi” for https://wiishlist.com/story19437461/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39