Buying a used 4x4 is fun. Not only do you have a reason to pick up the latest classified rags, but you also get to kick a lot of tires and talk shop with your buddies about the best vehicle for off-road use. As a first step, it's good to know what you want, but you should also keep an open mind when you're shopping for a used 4x4. I was desperately seeking a Jeep Wrangler TJ, but I found a '97 Jeep Cherokee XJ that I just couldn't live without.
I usually begin a pre-purchase inspection with a quick walk around the vehicle, including a glance at the roof, looking for any body damage or visible rust. I try to do this first because my first instinct is to hit the pavement and crawl underneath, and I usually forget such an easy step later on in the inspection. I actually missed some major roof damage when I bought a fullsize pickup a few years back, so I'm careful not to make this same mistake.
After the walk-around, I jump into getting dirty and start tinkering with everything. While underneath a prospective purchase, I first check to see what I'm looking at. With the Cherokee I was well aware of what drivetrain components would be and might be installed, but sometimes you'll find something nonstock that may or may not have been installed properly. It could also be a sweet upgrade that might cost you a bundle later on. Educating yourself on the stock components and available aftermarket upgrades for each model-year vehicle you are inspecting for purchase will offer the best results in getting the most for your money. It's also a good idea to educate yourself on the registration and smog laws for your state before buying a used vehicle. For some states, California in particular, you may end up paying big bucks to make your newly acquired vehicle legal.
Under the hood, you should check all fluids, looking at fluid condition and level, although you can't really tell fluid condition visibly unless it's seriously old or burned. Most fluids will typically darken with use and won't always be an accurate indicator of age or neglect. Fluids that appear dark brown or grainy with signs of sediment should be replaced and could be a sign of a larger issue. Signs of moisture in any of the fluids can also indicate a larger problem. As a general guide, transmission fluid is usually red, power steering fluid is often clear or pinkish-red in color, coolant is almost always green, and engine oil and brake fluid are typically light amber to brown in color.
If, after all this, you're satisfied with what you see, take the prospective purchase for a test drive. Leave the radio off and listen for anything out of the ordinary, i.e. grinding, clanking, clunking. Shift the transfer case and the transmission through its gears while parked and then again through the transmission gears while under acceleration. Don't forget to test the brakes before you really need them. If the vehicle meets your requirements after the testdrive, then it's time to make an offer.