It's really none of my business why they have one. If they were using it to do something terrible (like take pictures of other students in school to embarass them and you were the teacher) then you would need to intervene and maybe take it away from them. A person in a coffee shop who doesn't even know the kid doesn't have anything to do with the sitaution. I really don't think a 4 year old NEEDS an ipad, but how is it having such a great impact on me if he does?
EDIT: I just wanted to add that the only reason I didn't have a laptop, cell phone and ipad as a kid is because it hadn't been invented yet. I had a desktop computer (which was in the living room), a discman, and plenty of nintendo stuff. That *was* the newest technology at the time. Now, instead of a discman, I have an ipod. Instead of a desktop I have a laptop and I have a cell phone. I will be 24 next month.