Question...
Are the valves in series or parallel?
I can understand why they might be in series because that way each one protects the other. In other words all 4 need to be open before any air is released. However the above says that they are in different parts of the plane which would imply long ducts to connect them all up. It implies they are connected in parallel.
If you had a plane with 3 valves then adding a 4th valve in parallel would increase the risk of loosing pressure accidentally but would reduce the risk of not being able to depresurise for landing. Which is a worse failure?