For words that only have 3 letters, there isn't any 4th letter. For instance with the word “insert”, there are no additional letters afterwards and so need to be the word "insert". The word “addict” is during the BIP39 wordlist however, you might have made use of “addi” for “addict”. https://bookmarkshut.com/story19675795/a-secret-weapon-for-bip39-words-to-numbers