I'm going to throw something out there that's not answering your question but I think fits what you're going for. Flights from Trenton to Nashville can be pretty cheap. Flight time is 2 hours. Assuming 2 hours to get to Trenton and get through security, that fits your 4 hour timeframe. The cost of flights isn't the same as the cost you'd spend on gas for 8 hours total of driving but it's not orders of magnitude higher.