When I was in a similar situation someone else on this forum suggested turning the rental into a weekly rental; this will usually result in a lower rate than 4 days at the weekday rate. You only needs 4 days and a few hours to have the rental switch over to a weekly rate. Just bump your return time to later on Sunday or Monday morning.

If you reserve this way but don't actually keep the car this long there is the possibility that when you return the car they will revert you back to the daily rate. Again, other people here have not had a problem with this - the return attendant keeps the lower rate. I would think that if they try to charge you more simply talking to a manager and saying your plans changed would solve the problem.

My plans actually did change before the rental, so I can't speak from experience with this, but I wanted to pass on the helpful advice I got.