Guys, his budget is $4k and you're throwing names like Audi and M-B at him?
Having owned about a zillion cars in my life, I would not buy anything German for $4,000. The repair cost to vehicle value ratio will eat you up. Look at $1,000 plus for rotors and pads. Anything electrical - you are screwed. The worst car to buy is an aging VW-Audi product. You'll be reaching into your wallet every time you turn around.
The problem with Japanese used cars is the price. They hold their value a little too well.
Take a look a Jeep Cherokee. Do not confuse it with the V8 Grand Cherokee. The V8 Grand Cherokee is a P.O.S. The 4.0l Cherokee is a tank.
Considering that you'll be driving the car infrequently, you want simple, stupid technology. A cast iron, inline 6 designed in the 80s fits the bill. AMC/Chrysler made 5 million of the things.
http://en.wikipedia.org/wiki/AMC_Straight_6_engine#4.0