Here is a list of recommended books for 4th graders.
The Magic Treehouse Series by Mary Pope Osborne
The Lost Series by Margaret Peterson Haddix
The I Survived Series by Lauren Tarshis
Nightsong by Ari Belk
The Percy Jackson Series by Rick Riordan
The Tale of Despereaux by Kate Dicamillo
Harriet the Spy by Louise Fitzhugh
Bone by Jeff Smith