Having lived through the 1970's recession, I saw people make very foolish vehicle purchase decisions based on the sudden increase in the cost of gasoline, virtually giving away expensive cars and dramatically overpaying for cars such as the Volkswagen Beetle. Surprise;-history is repeating itself.
Before you go out and buy a used higher-mileage car, consider this; the buying frenzy is in full swing, and cars such as the Honda Civic and Accord are selling for a lot more than can be justified for the saving that you will experience at $4 gas. Should the price of gas drop, the difference could be even more dramatic.
Here's an example; I found two 2004 cars for sale, a Honda Accord EX and a Buick Park Avenue. The Accord gets great mileage; 24/34, while the Park Avenue's mileage belies its size, at 20/29. An eye-opening statistics; the Buick originally sold for $17,500 more than the Accord. Now, however, as a used car you can pick it up for $3,000 less. Even with the lesser mpg, how long would it take for the Accord's mpg advantage to make it the shrewder purchase? Take a look at the numbers:
Ignoring maintenance and insurance costs, on mpg alone you could drive the Park Avenue for 100,000 miles and your total cost, even with the lesser mpg of the larger car, would still be less than the Accord.
This is a clear example of what happens during a feeding frenzy. My advice; look at the TOTAL lifetime cost of each vehicle you consider, including the one you are aching to dump at a huge loss. You might well be surprised to discover taking a $10,000 loss in the trade will never be recovered by the fuel saved.
Note: I realize that the asking price for these used cars could be negotiated down, but probably this would increase the difference in sales price, as the Accord is more popular.