Check out http://coreknowledge.org/CK/index.htm
Hirsch's core knowledge website. Especially look at "Lesson Plans" on the left side of the screen. The list of books for 4th graders fit what GS9 read this year on his own, honestly I can't imagine many other 4th graders reading them on their own, unless they are abridged versions. But I really like his approach to education.