The Quad crank valve was designed to replace the top drive valve with a single body valve with two seperate valves built into the body.  The purpose of  the Quad crank valve is to avoid any rig down time due to valve failure.  When a failure occurs to the upper section of the valve, there are some linkage that will attach the sleeve to the lower cranks on the Quad valve and the section of the valve can then be remotely operated and can continue with drilling until it can be changed at the first scheduled maintenance without any rig down time.