For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist however, you would have applied “addi” for “addict”. In this https://apollobookmarks.com/story18765252/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition