4x4: the first digit refers to the number of wheels and the second digit to the number of driven wheels (dual wheels count singly). Thus 4x4 is a special case of all wheel drive where "all" is four.

4x2: four wheels, two driven - ie. most cars.

Part-time four wheel drive can also be described as selectable 4x4 or 4x2.

6x4: many semi-trailer tractors have 6 wheels, four driven.

6x6: six wheels, all driven etc.