For words that have only three letters, there is absolutely no 4th letter. Such as with the word “incorporate”, there are no additional letters afterwards and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. This https://ok-social.com/story4298150/5-simple-techniques-for-bip39-seed-phrase