4x4, also known as 4-by-4 or four-wheel drive, refers to a system where a vehicle's engine power is sent to all four wheels evenly so that they are all moving at the same speed. Made famous by the classic Willy’s Jeep, this type of set-up is popular with outdoor and off-roading enthusiasts and is common on trucks and SUVs.
When it comes to how a vehicle moves its wheels, there are plenty of options: front-wheel drive, rear-wheel drive and all-wheel drive. The 4x4 option often gets grouped in with all wheel drive, though the two systems differ.
In a true 4WD system, the front axle and rear axle travel at identical speeds – this means that all four wheels are rotating at the exact same speed. This is ideal when traveling over very rough or uneven terrain at slow speeds, such as rock-crawling. But it does not work as well when traveling around corners. When a vehicle turns, the outside wheels are traveling at a different rotational speed than the inside wheels. If you are turning a four-wheel drive vehicle, you will undoubtedly notice the wheels skip as the outside wheels try to play catch up with the inside wheels, particularly on dry pavement.
In an AWD car on the other hand, the system allows the engine to send more power to the axle or wheel that are not slipping, meaning that they can travel at different speeds. For this reason, many modern all-wheel drive systems are often referred to as electronic all-wheel drive. This means that an all-wheel drive car can maintain traction with all four wheels around corners and at higher speeds than a traditional 4x4 system.
Whereas AWD cars are always in AWD mode, a four-wheel drive vehicle usually has an on-demand system that requires the driver to engage the four-wheel drive system. When not in four-wheel drive, the vehicle stays in two-wheel drive mode (usually rear-wheel drive for a truck or SUV). Four-wheel drive vehicles often have two sets of four-wheel drive gear ranges: 4 low, for lower speeds and climbing grades off-road, and 4 high, for higher speeds or on-road use.