Fourth Clutch
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fourth Clutch fluid is exhausted at the 3-4 shift valve and the driven support ball check valve. Fourth clutch return spring pressure moves the fourth clutch piston and releases the fourth clutch.