4x4 Four by Four Four wheel drive 4WD 2WD

4x4 Four by Four Four wheel drive

The first digit refers to the number of wheels.
The second digit refers to the number of driven wheels.
4x4 (four by four):
A vehicle with four wheels that's powered by four wheels (4WD)

Four wheel drive axle arrangement gearbox
Four wheel drive and two wheel drive axle arrangement gearbox
Axle and housing working diagram
Axle and housing working diagram