You are adding four days, not three.

Hertz system is programmed to give you lowest rate based on your rate plan. Depending on the rate plan. Usually, three (sometimes two) additional hours is more the one extra day and system defaults to one extra day. I have no personal experience with four extra days, but it is possible that four extra days is more than one extra week and system defaults to one extra week. I have never rented Hertz for more than one week.

More than once, I have had to take a later flight in order to keep Hertz car for four days and three hours and pay for lower weekly rate instead of higher four day rate. As your booking is for more than one week, it is possible that you qualify for two weeks rate instead of one week and four days. Have you tried booking two separate reservations, one week + four days, to check?