First of all, just throwing those dates into Orbitz I see lots of available cars in the $30's per day, so that's good news. Your starting point isn't all that bad: definitely not bad enough to reconfigure a trip's plans just to shave a few bucks off the car rental. You just need to find some good codes to bring it down a bit.

My first option would be to go the other direction: make the rental slightly longer than four days. That'll kick in a weekly rate, and then you can drop in a good promo/discount combo and get some percent off, plus either bonus credits, upgrades, free gas, or additional dollars-off. You starting point is $156/wk with Hertz - a little higher for bigger cars, and maybe lower with other agencies.

If you need a specialty car, Frenchie has hit on a good option: Priceline. The Wed-Sun scenario is a rare one where Priceline usually does yield a pretty good option - for stuff like convertibles or minivans, you can save a lot of money for a minor amount of hassle.

I've done a car swap when needed - if I was already planning on being right near the Hertz location and the rate differential was significant. But my guess is that you can work the codes on this one to get it into a tolerable range, and then go about your previously planned trip.

There are long threads with codes on both the Hertz and National boards. Consider them both. The attractive Hertz promo at the current time is large amounts of WN credit with weekly rentals, or you can do the usual dollars-off, etc. The National folks have uncovered Amex and Carlson Wagonlit codes that seem to be stackable with other things like Free Days and big bonus miles.