You haven't paid for anything yet... The 4 billion is for development and infrastructure. You're paying for the pad modification/construction, manufacturing of first articles, flight certification and testing. Normally, if this were a commercial airliner, Boeing would pay for this themselves. They would recoup the cost over the couple hundred planes they constructed and sold. Since there is no business case with and end result allowing them to recoup the cost, they're having the primary customer (NASA) pay the Non Recoverable Engineering costs (NRE). This is standard practice in industry (any and all industry).
Once certified, NASA, as a customer, will be buying seats on the CST-100, as a service. They're not buying the rocket for their exclusive use; think of it as buying seats on a commercial airliner. NASA will be the primary customer initially; but, not the only customer. Same thing happened with the commercial airliner industry -- in the very early days, one of the major customers was the government and a buyer of cargo space was the U.S. Mail service.
ISS is not going to be the only destination in the future. Bigelow Aerospace plans to launch a habitation module or two in the next few years. They already have a contract with Boeing to use CST-100 as a transport. It's just the beginning...