The controls shown above are for the Series 2 and early Series 2A Land Rover. Later Series 2A's have a different ignition switch with separate lighting controls. The instrument panel for the Series 1 is in the same location but smaller, and for the Series 3 it is behind the steering wheel.
All Series II and III Land Rovers have selectable 4-wheel drive. On standard road surfaces 2-wheel drive is necessary (else transmission damage can occur).
The yellow knob controls instant access to 4-wheel drive. You simply push it down until it locks in position. This can be done whilst driving. The result is 4-wheel drive using the main gearbox. To revert to 2-wheel drive the vehicle needs to be stationary, the red knob is pulled rearwards and then returned to its original position. The yellow knob will automatically pop up again.
Sometimes the Land Rover is required to do hard work over very rough terrain or climb very steep inclines. In these situations low gears are required and these are only available in 4-wheel drive option. To select low ratio gears the vehicle must be stationary and the red knob pulled towards you. To disengage low ratio then the red knob is pushed away from you, but again the vehicle must be stationary.
In summary, the yellow knob can be operated on the move but the red knob cannot.
If you see a Series Land Rover with a 4th gear lever then somebody has fitted an overdrive as an optional extra. This would only ever be used in 2-wheel drive on good road surfaces as an aid to fuel economy.