Car rentals for 4 days may be as much/more than for a week. We paid $750 for 20 days in France for a Toyota Corolla--same class as Ford M. Perhaps they didn't rent from US, or use any special coupons?

Yes, $600 was a lot to pay for four day's rental, but the actual daily rental was only 40 GBP for an intermediate sized, automatic, air-conditioned car. The additional charges included tax, location fee, 8 GBP daily for additional driver, personal liability insurance that I think was around 5 GBP daily (we declined the CDW and theft), and some other charges that cannot be avoided. It also included gasoline as we returned the vehicle with only three-quarters of a tank -- huge error in not refilling before we turned in the car. Based on my experience, renting for under a week can be costly.