For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any far more letters afterwards and thus should be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have made use of “addi” for “addict”. https://hindibookmark.com/story20491506/considerations-to-know-about-bip39-word-list-pdf