I don't think you get that. The 5th night free is when you are using points. This, as I understand it, would be 5 free night certs. When you used free night certs, you are not using points, so the 5th night free would not apply. Same as if you had a free night cert from a credit card now and booked 3 nights with points and one night on a free night cert, you would not get a 5th night free.