How do you tell a 4-year-old who the Buddha was?
A 4 year old can be told that the Buddha was a man who wanted to understand how to be happy and free and he worked very hard to understand how to be truly happy and free. Then he taught other people what he learned. 

For many years the Buddha taught: 


Sometimes people feel sad.
Sometimes the thing that makes people sad is not getting something they want or getting something they don't want.
There is a way not to be so sad about not getting what you want or getting something you don't want. 
The way is to not think so much about what you want at all, but instead think about how you can be kind and helpful to your family, your teachers, your friends, other people, animals, bugs, and everything that lives. :)