Knoxville and Nashville are only 100 miles apart. At 70 MPH, that's an hour & ½, plus however long it takes to get from the Interstate to a parking space. It's pretty close to I-40, so that's not a problem. Four hours from Knoxville will get you to Memphis!
But if you're staying in the Gatlinburg area, that could easily add another hour to your trip. I would recommend taking US-321 from Pigeon Forge west through Maryville to I-40 west of Knoxville as an alternate. It saves you from having to deal with Knoxville.
There's a Mexican restaurant two doors down from the Exit/In. They have a parking lot right behind them, but it's for customers only. Do like we did and get there early, park in their lot, eat at the Mexican restaurant (thereby being customers), and then walk down to the concert.