How come I have an Iphone 6 plus. But because I use sprint I can't go faster than 3G?

A few people I know with sprint get 4G. But the people who I speak to over the phone claim 3G is the fastest known to man. Probably because they are copping out on customer service. But why am I in this situation? How can I get 4G? Seriously even a dumb answer is better then being sold on slow.

Ask sprint.