Looking for high level books for 4th graders

I have some very high level readers....several are on an 8th grade level. I'm looking for higher level reading, but without the iffy young adult content. One student in particular, is actually younger, having skipped a grade. He's already read the classics (Twain, Kipling, RL Stevenson, etc.)

So far, I've come up with Rick Riordan, Redwall, Artemis Fowl, Michael Scott Nicholas Flamel series. Any other suggestions?

MrsHouseLibrary suggests:

Gregor, The Overlander by Suzanne Collins
The Eldest series by Christopher Paolini

Then she told me to try this site: Literature Map
It wants a first and last name of an author, and gives you a cloud of other author's names. In theory, the ones that are closer to the one you entered would be the most likely other authors. There is no explanation for the placement of other authors. I entered Rick Riordan, and Eoin Colfer showed up twice. The one on the left side appeared in the vicinity of authors of who write books for 3rd through 5th grade levels, the ones toward the right, I associate more with older kids. So, there is some criteria applied, but they keep it to themselves.
Clicking on an author's name reshuffles the map. There's nothing to link you to a list of books.

Terry Pratchett and Neil Gaiman and Susan Cooper come to mind...and for a few specific books, London Calling by Edward Bloor, White Darkness, by Geraldine McCaughrean and the Pagan series by Catherine Jinks.