You can't very well buy used. But you should buy used kids crap whenever you can. I would get a pretty pink one from Walmart, which you may have to do some assembly on in the basement. They are rugged enough to survive your 4 yr old, and your sisters kids and the in laws kids. That bike will be passed through 10 kids without showing the first sign of wear. Why? Because the most a kid will ride it is one season before they grow out of it and are ready to get on a larger bike. My girls probably went through 7 bikes between the ages of 5 and 12
