My 4.5 year-old likes Shel Silverstein's Giraffe and a Half, the Magic Tree House series, the Young Cam Jansen series, and the Thea Stilton series.

A word of warning on the Magic Tree House series. My son did fine with them, but I was not prepared to discuss issues like slavery, the deaths on the titanic, the unpleasant nature of war, etc. The series gets into some heavy topics pretty quickly. All things considered, I feel like reading them was a positive experience, but not all kids would benefit from reading them this early.