My son also went through a fascination with really big numbers when he was four. My husband is a mathematician and they had a lot of fun discussing this. Our favorite book on the subject is Really Big Numbers- the first book for kids published by the American Mathematical Society.

http://www.amazon.com/Really-Number...56231&sr=8-1&keywords=really+big+numbers

I can't recommend this book highly enough. My son learned about combinatorics, exponential notation and graph theory all from this book. It touches on so many fascinating concepts and kids - and adults- with many levels of math literacy can all appreciate it.