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 :)