What is the purpose of the Four-Circuit Protection Valve (Vierkreisschutzventil) in a heavy vehicle compressed air braking system?

1. To distribute hydraulic fuel to four engine cylinders
2. To control the four cabin speaker volume levels
3. To ensure that if one brake circuit suffers a pressure drop or air leak, the remaining circuits (Circuit 1 rear, Circuit 2 front, Circuit 3 parking/trailer, Circuit 4 auxiliary) maintain operational safety pressure
4. To switch between four different transmission speeds