In the U.S., if you reserve a car for a weekly rate (typically a minimum of 5 days) and return it after 4 days, the rental agreement usually allows the company to reprice the rental at an often-much-higher daily rate.

Nonetheless, I think it would be significantly easier to return the car earlier than try to extend it later; the only issue is the cost. Make sure you read the rental agreement to understand what the impact will be.