For words that have only three letters, there isn't a 4th letter. For instance with the word “incorporate”, there isn't any much more letters afterwards and so needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://bookmarkmoz.com/story19022933/a-review-of-bip39-passphrase