You'll no doubt have come across various terms relating to 4x4 such as AWD, 4WD, Quattro, ALLGRIP, ALL4 and 4Motion. As confusing as these terms can be, they all relate to two different types of 4x4 cars: 4-wheel drive or All-wheel drive, and they are quite different from each other.
4WD or 4-wheel drive cars, like the Suzuki Vitara ALLGRIP and Nissan X-Trail 4WD spend the majority of their time running as a 2WD car unless the need arises. With this type of system, the 4-wheel drive mode doesn't kick in until it detects slipping on one of the two wheels powering the car, thus allowing the car to grip and remain stable. Modern cars often have mode selectors which allow you to turn the 4WD on permanently, for such instances as driving in snow where slipping could occur frequently.
Cars which come under the AWD category like the Subaru Forester, Outback and XV, are in permanent 4-wheel drive mode. This means that all four wheels are powered 100% of the time. This is particularly useful if the driver is negotiating rugged, off-road terrain. Professions such as farming often require this capability, which is why you'll often see them in vehicles like Land Rovers which are permanently 4-wheel drive!
Regardless of whether the car is permanent 4-wheel drive or not, 4x4 cars give added stability and grip in situations where a standard 2WD car may slip or get stuck. Where two wheels are only at work in a 2WD car, all 4 wheels work together simultaneously in a 4x4 to move the vehicle. If you live in a rural location with muddy tracks, or your job involves frequent off-road trips - then a car with 4x4 capability is likely to be a priority for you.
Where performance is a factor, 4WD cars are quicker off the mark than a standard rear or front wheel drive car because all the power is applied to all the wheels from the get go, so there is no lack of traction. Combine this with launch control which many performance cars also come with, and this is one of the reasons that these heavier cars with lower bhp can outperform lighter, higher-powered 2WD models.
To power four wheels instead of two requires additional vehicle parts, and in the case of 4WD cars (rather than AWD), it requires additional technology on board to control when the system kicks in. These extra parts and technology come at a cost and weight meaning that 4x4 cars are more expensive to purchase and run. The extra weight and the need to power twice as many wheels also means an increase in running costs. Though the latter can be circumvented to a degree in 4WD cars thanks to their clever selective mode technology.