No Fourth Gear

WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

Checks	Causes
4th Clutch Assembly	Inspect for damaged or worn clutch plates. Inspect for damaged piston, piston dam, dam retaining ring, and clutch spring. Inspect for improper selective 4th clutch backing plate retaining ring. Inspect for cut, worn, or damaged turbine shaft fluid seal rings. Inspect for damaged or distorted turbine shaft.
Control Valve Upper Body Assembly	Inspect the 12-3-4 clutch regulator valve and spring for wear or damage.
Output Shaft Assembly	Inspect for damaged or stripped internal splines on the output shaft assembly.