Take the Metro bus instead of Caribe Tours. Takes about 3 hours. Caribe Tours stops twice in Santiago and once in La Vega, with some small stops along the way. You will pay about 40 Pesos more but it is much faster and better.
You can get it near the Tropical Supermarket. Ask anyone for directions.
Both bus trips are a good 3 hours and 30 or 40 minutes. They are virtually the same cost and time. Both buses are very very cold - air conditioners blast. When I choose one over the other it is almost always about where I want to either get the bus in SD or get off the bus in SD. Metro terminal is in Piantini area and Caribe is more in the core area - don't know what it is called.
4,500 RD seems a bit high for this trip. We just had a taxi go from Puerto Plata to Juan Dolio (45 minutes the other side of SD) and return for 6,000 RD. However a night time trip will cost more.
Make sure to negotiate as Canada Bob says. It is a beautiful trip and should take about 3 to 3 1/2 hours depending on time of day and traffic conditions.