For words that only have three letters, there is no 4th letter. As an example with the word “incorporate”, there aren't any extra letters afterwards and so need to be the word "insert". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. bip39-phrase.com is https://myeasybookmarks.com/story4410893/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting