The chipmunk is actually a .22 built just for kids.
I don't think that 4 is too young. The childs age isn't nearly as important and the parents brain (or lack there of).
First, Harry Carry has a great point.
As I haven't seen a pellet gun of the necessary size, you might think about a chipmunk (or one of the other brand versions) and CB caps in .22. They are basically a .22 caliber ball and just the primer to power it.
The length of a marriage is inversely proportional to the amount of money spent on the wedding.