If your son is the one asking for them then maybe he is ready. They run very small. I got a pair for my 6 year old daughter and had to get Heelys a size bigger than what she normally wears. My 4 year old son wears an 11 and I had to get him a 13. He begged me for them and so I realized just because I was scared for him and thought he wasn't ready, didn't mean he wasn't. He held on to me and the shopping cart everywhere we went until he got it down. Now he is a pro and just takes off.
As far as excercise. OMG! I can't believe the strength in your feet and endurance in your legs you need to keep your feet up. It is not easy at all to do anything in just your heels. You try it. Your feet get tired fast and you want to put your toes down. So I wouldn't call it laziness, cuz it takes alot of work to roll around.
My kids wear the wheels, I would say half the time. There are alot of times besides just school that they just wear the shoe without the wheel. This saves me money on having to buy them skates too. My son goes outside and just skates.
As far as the kids being rude in the store and weaving in and out of people. I don't know about your 4 year old but mine is constantly getting in the way of everyone anyway. He never watches where he is going and I am constantly telling him to watch out, get out of their way. Don't touch that,you are going to knock it over etc....
He is too big to fit in a shopping cart now. Poor kid can't get his legs through the holes and I need the big part of the cart for the groceries, or what have you, so he has to walk.