If you're going to be in the area ....
Here are some things you should deffinetly check out!
Playa El Quemaito
Rio San rafael
Playa Los Patos (take a dip in the very cold lagoon!!)
Polo Magnetico (you will see your car go "up" the hill by itself!!)
Lago Enriquillo (check out the Taino petroglyphs called Las Caritas)
Las Barias
For these things you don't need a 4x4, but if you decide to go to Pedernales you should then get a 4x4 and go to Bahia de las Aguilas.
On your way to Barahona stop at Bani and check out the dunes and Las Salinas.
Let us know how it goes!
jojocho