So I've been having a lot of fun with Kate's riddles, and I might've reached the end? Am I supposed to know a code when she asks "what can I help you with?" "???" "you're looking at things backward?" ... what secrets is she holding
You need to remember the four letters you gave her for all four sets of her riddles. She specifically says at the end of that last set, that you've gained knowledge that will be useful some day, aka her final riddle, the question marks.
Enter them in order, and use that clue - "backwards" - and you'll have your answer.
Happy to just reveal it if you want, it's just more fun and rewarding if you figure it out on your own first :)