It depends on the terms of your rental. YOu can return whenever you want, but that's not to say that you'll be charged 2/4th's of the price. If your price is a special price that requires a sat night, or a minimum 3 day rental, or uses a coupon carrying certain restrictiions, you would have to read all the fine print to see what you may be charged. You may be able to get an indication by going on line and booking a 2 day rental and see what that amount is. When will you know how long you'll need it for? You could always hold both the 2 day and the 4 day reservation and cancel when you know which you'll want.