Nope, my guess is that National will not return the 4th free day credit automatically. You may be able to call reservations and tell them you want to return your car one day early, can they adjust the reservation and give you credit. But once the car is checked out, it's in their hands.

My preference would be just book a 3 day rental, then if you need a 4th day, call reservations and ask if you can add one more day. I've been able to extend my rentals using free days from my account. Worse comes to worse, book another car for one day 24 hours in advance and you're gaurenteed to get a car. Good luck.