Four-wheel drive, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. While many people associate the term with off-road vehicles and Sport utility vehicles, powering all four wheels provides better control than normal road cars on many surfaces, and is an important part in the sport of rallying.
http://pt.wikipedia.org/wiki/4x4
http://en.wikipedia.org/wiki/Four-wheel_drive

In standard two wheel steering vehicles, the rear set of wheels are always directed forward therefore and do not play an active role in controlling the steering. In four wheel steering systems, the rear wheels can turn left and right. To keep the driving controls as simple as possible, a computer is used to control the rear wheels.

http://www.wisegeek.com/what-is-four-wheel-steering.htm

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