That's almost correct, the 4G speed for data never changes. They limit the quality of video, which has nothing to do with data speed. They cap video quality at DVD as opposed to HDDVD or Blu-Ray. You have to pay more for high quality HD video, which is playing favorites to their service and by requiring you to pay extra for that service it is a very clear breach of net neutrality.
I was under the impression that only the content provider can limit the quality.
For example here are the speed recommendations for Netflix,
0.5 Megabits per second - Required broadband connection speed
1.5 Megabits per second - Recommended broadband connection speed
3.0 Megabits per second - Recommended for SD quality
5.0 Megabits per second - Recommended for HD quality
25 Megabits per second - Recommended for Ultra HD quality
Netflix, as the content provider, will adjust your stream quality based on the speeds of your connection. So if Sprint is only allowing 1.5 megabits per second, Netflix isn't going to allow an SD quality stream. So it is Sprint that is controlling the speeds your data transmits and Netflix adjusts the quality of the stream accordingly.
I could have this all wrong though. That's just the way I've always understood it.