I just dealt with the same issue in MCO and probably would have missed my flight because of it, but my flight ended up being delayed.

I kept my car just over 2 hours beyond the 4 days: 1 hour for the free hour they give you, and 1 hour because I was concerned they would change me from the weekly rate to a daily rate at 4 days + 1 extra hour. I tried calling Hertz at MCO to see how they would handle a return at 4 days and 61 minutes, and the guy said I could bring my receipt to customer service for an adjustment if I returned early and the rate changed. But I wasn't convinced he understood what I was asking and didn't want to take any chances.

I had also put the reservation time earlier than what I needed, without my flight info, but the rental wasn't ready when I got there and they had to put the current time on it.