3-4 Shift Valve
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
D3 fluid pressure assists spring force to keep the valve in the downshifted position against the signal A fluid circuit. In this position, the valve blocks 3-4 signal fluid and the 4th signal fluid circuit is open to an exhaust port past the valve. Therefore, with D3 fluid pressure assisting spring force, Fourth gear is hydraulically prevented.