footnote: I don't know about the multispeed model for a 4 year old. I would've bought that later and use singlespeed for now... but if you've already bought it, I would put it in a middle cog and tell my son not to mess with it. (not sure if that would work or not!)
That was going to be another question. They have a single speed model called the Kazoo which is $50 cheaper but it's basically the same bike.