Besides from the obvious fact that all four wheels are driven and thus giving more traction, there are other important differences between a two-wheel and a four-wheel drive:
Low transfer gearing: Needed to take steep, difficult terrain slowly and carefully
Center differential lock: Must be locked immediately when driving through difficult terrain, because if it not locked and one wheel loses traction, all power will be distributed to this (now free-running) wheel. If the centre differential is locked, then two wheels have to lose ground contact for this to happen, and it must me one wheel from the front and one from the back.
Axle differential locks: Do not lock on tar (excessive tyre wear). May create difficulties on flat terrain there the surface is slippery, but traction almost the same for every wheel (e.g. sand), understeer. The rear lock always has to be engaged before the front lock!
Rule of thumb: Engage the center lock immediately when leaving tar, engage axle locks only to get out of difficult situations, do not lock front if you need precise steering.
Corrugation: On gravel roads sooner or later corrugation will build up. This will result in shaking if you drive more than about 20 km/h. At a higher speed (70 km/h) it will get better, as the car skims from wave to wave.
River: Carefully watch other cars driving or (if safe) walk through to see how deep it is before. If the engine block is hit by the water it will be cooled down very quickly, this might result in cracks. Try to avoid water deeper than the hub of the wheel.
Sand: Release pressure out of the tyres before hitting a sand track, 0.8 to 1.0 bar, depending on the load of the vehicle. Do not try to steer against resistance, the car will usually seach his track. Do not shift gears. Do not brake as this will build up small hills of sand in front of your tyres, making it difficult to start again. If you get stuck do not try to accelerate as this will only dig you more into the sand. Try driving backwards instead. Use high ratio because the additional momentum given by the low transfer gear will cause wheelspin.
It is highly recommended that you carry a recovery kit when traveling offroad. A simple kit can mean the difference between getting yourself out of a tight spot within an hour or being stuck for days waiting for help to arrive.
A basic recovery kit should contain at least the following:
Gloves - Most injuries occur while trying to extract a stuck vehicle.
Tree protector - Do not use your tow straps or winch cable directly around trees, as this might cause damage.
Snatch Block - To double you winching power or allow your vehicle to be pulled even when there is no space for another vehicle in front of it.
D-Ring Shackles - To attach things together.
High lift jack - In most places where you might get stuck, the standard jack will just not be high enough.
Always ensure that the equipment is from a reputable manufacturer and of good quality. When suck in the mud 100km from the nearest town, a working snatch block is worth a lot more than a broken one with a 12 month no questions asked exchange warranty.
Due to quality concerns it is generally better to build your own recovery kit by purchasing each component individually, than to buy a complete all-in-one recovery kit.
fuel filters: Especially when driving in third world countries, as the fuel is often contaminated with dirt and water
epoxy putty (to seal big holes in the radiator)
self tapping screws (various sizes)- used with epoxy putty to seal holes in fuel tanks, fuels lines, brake lines etc.
cork (larger than your oil drain plug) - situated at the lowest part of the engine the oil drain plug is a prime candidate to be ripped out by boulders in rough terrain. When this occurs the drain plug threads are normally also damaged and you will not be able to fix a replacement plug; cork and epoxy can create a very good temporary seal until you can have the problem professionally seen to.
First check the cooling water level. If it is low, refilling does not help too much, because as you have lost it during the last short period of time (you have checked it before you started your journey, haven't you?) you will probably lose it again very soon. So you have to search the leakage. Check the tubes from and to the radiator, and the radiator itself.
Bigger punctures in the radiator can be repaired with epoxy putty, smallers by putting egg white in the radiator. But be sure to clean this as soon as you can. If you have a leakage in the radiator hose put a (used!) chewing gum on it and fix it with some duct tape.
If the cooling water level is ok, but the engine is still running hot, something might prevent the water from being cooled. Check the fan belt, is it loose or broken? Have a look at the radiator, is grass or seeds disturbing the airflow?
Another common problem is a defective thermostat. The thermostat is responsible for by-passing the radiatior as long as the engine is cold, allowing the engine to reach its optimum operation temperature quickly. But if it is broken, the cooling water will not reach the radiator and the engine heats up too much.
To work around this problem without having a spare thermostat just remove it. Then the water will always flow through the radiator and your problems should be gone. Replace it it soon.
Organized tours are becoming quite popular. It ensures that there is at least one experienced driver in the group and that there is always someone to assist in recovery if a vehicle gets stuck. Many people also prefer the social interaction of a larger group rather just one or two vehicles travelling on their own. Some tours may even be fully catered with regular meals provided.
This is a usable article. It touches on all the major areas of the topic. An adventurous person could use this article, but please plunge forward and help it grow!