4x4 has two meanings. It describes a transmission system which has four wheels, all four of them driven. (A normal two-wheel drive car would be described as a 4x2.)
4x4 is also used for a type of vehicle, which has a 4x4 transmission, and which provides off-road ability – such as a Land Rover Defender.
On-road four-wheel drive vehicles such as Subarus and Audi quattros have 4x4 drivetrains, but would not usually be described as 4x4s – because they are essentially on-road vehicles.
