For words that only have three letters, there is no 4th letter. For example with the word “incorporate”, there isn't any extra letters afterwards and thus must be the word "incorporate". The word “addict” is from the BIP39 wordlist however, you would have used “addi” for “addict”. Finding a single https://top100bookmark.com/story19190853/the-basic-principles-of-bip39-passphrase-generator