The term four wheel drive in the UK is used to describe a car where four wheels are driven usually via viscous couplings or other means of distributing the power, such as computer controlled clutches.

The maximum power distribution to each wheels should ideally be proportional to both the weight on each wheel which depends on the mass distribution of the vehicle, the weight transfer, and effective friction.

Except on low friction surfaces, or on exceedingly powerful cars four wheel drive generally gives lower performance than rear wheel drive due to the extra mass required, for the same engine power.

Rally cars are both exceedingly powerful and run on low friction surfaces so use four wheel drive wherever possible.

See the North American term all wheel drive which describes the same concept.