For a light read that is still fairly good for information, H.W. Crocker’s PIG to the Civil War is not bad. It might be a bit difficult for a 4th grader, but the PIG books are designed for a lower reading level.
This is a great book on Lee, written for that level in 1898: http://books.google.com/books?id=UVtLAAAAYAAJ&printsec=frontcover&dq=the+life+of+robert+e+lee+for+young+readers&hl=en&sa=X&ei=uXMtUceNAYuw8AT92oHQDQ&ved=0CDwQ6AEwAQ#v=onepage&q=the%20life%20of%20robert%20e%20lee%20for%20young%20readers&f=false
I am not sure what type of report he needs to do. If it is a simple book report, then the PIG book is out. The Lee book would be more appropriate, though old.
She also wrote a similar book for Thomas J. Jackson, also good.