For words that have only 3 letters, there is not any 4th letter. One example is with the word “insert”, there aren't any extra letters afterwards and so should be the word "include". The word “addict” is from the BIP39 wordlist however you would've utilized “addi” for “addict”. A seed https://thebookmarkking.com/story18884309/the-invalid-bip39-phrase-diaries