Of the things my son enjoyed hearing read aloud at 4, these are the ones that are on approximately that reading level:
Winnie-the-Pooh and its sequels by A.A. Milne
Ramona the Pest by Beverly Cleary
Little House in the Big Woods by Laura Ingalls Wilder. This is an easier read than other books in the series and has lots of pictures.
Many books by Ruth Chew, all about kids who discover some kind of magic. Some favorites are The Wednesday Witch, Magic in the Park, and The Witch's Garden.
Not sure what reading level these are, but "Jenny and the Cat Club" and other books in this series by Esther Averill are great early chapter books. We just discovered these and thought I'd mention them because I haven't seen them mentioned before. My little girl is very sensitive to scary stuff, but these are very appropriate. I also find them infinitely more charming than the Magic Treehouse series. I know the Magic Treehouse books are very popular and educational, but something about them--I can't quite put my finger on it--bores both me and my daughter.
Also, is he into non-fiction? Mine loves non-fiction and it seems easier to find age appropriate non-fiction books for preschool aged kids, IMO.
With the Olympics coming this winter, there are some great little books
DK, Eyewitness, and National Geographic have good quality non-fiction.
My Fathers Dragon, Moongobble & Me, Bink & Gollie, Elephant & Piggie (short and easy, but funny!), Binky the Space Cat (graphic novel), Books by Bill Peet, or magazines would be great for that age! Ranger Rick, National Geographic Kids, Ask, High 5, all are good.