You said you may need the car for 3 or 4 days. Rental agencies generally charge you for an extra day once you go over 2 hours. Keep the car for 4 days plus 2 hours and you might be charged for 5 days and qualify for your weekly rate.
Otherwise reshop your rental. A different agency/code/coupon may give you a better rate then the rate you'll wind up paying if you don't qualify for a weekly rate with your current reservation.