The other night my 4 year old picked out a couple of books to read before bed that had a science theme to them.
1. Biggest, Fastest, Strongest
This book discusses a few of the "extreme" animals in the world in terms of their size and/or what they can do. For example, what is the largest animal in the world? Smallest? Tallest? Fastest? The book does a great job introducing young children to these animals and has sizes to scale in the book, comparing the animal to a human.
2. Magic School Bus Lost in the Solar System
Overall this book does a great job introducing kids to the Solar System through a fictional story. Granted, it does a horrible job explaining the asteroid belt. It gives the impression that the asteroid belt is loaded with asteroids that you must dodge to get through when in fact there are millions of miles between any two asteroids. But if you can get through that part, it's a cool way to introduce kids to space.
There are plenty of other great books out there that introduce kids to aspects of science, but these were the two my 4 year old picked out last night.