No, they don't give you back $4000. They get $5000 from 1 million members and have to justify spending $4 billion. That savings gets passed back to you as a lower rate the following year. The point of 80%, I believe, is to drive the "for profits" out of the industry. With only a 20% margin, once you factor in the overhead, they are going to have to be aggressively lean.
This is the end run around price controls, call it profit controls.