Common Sense Media has a great website that allows you to search books by age. It then shows you if the books are content appropriate for your child. It also explains why they are rated that way. (It also has an area for movies, apps, and games too!)
With the move to Common Core State Standards, it is imperative that children continue to advance in text complexity (how challenging a text is), especially in nonfiction. Below is a file of the top 100 nonfiction books broken down by ATOS level range (average ZPD). It also shows interest levels for lower, middle, and upper grades. Please note that not all content material on this list will be appropriate for forth graders.
Good Reads is a site where you can create an account via email or Facebook. After signing up you can see book reviews, see what others are reading, and find your book in stores and libraries. Below is a list of popular 4th grade books.