At approximately 1400 mountain time, a Vans RV-4 suffered a propeller failure during cruise flight and was damaged in the resulting forced landing. The pilot was seriously injured. The pilot said he was cruising at 8,500 feet when the airplane began to shake violently. He shut down the engine and, when the propeller stopped, noticed the trailing edge of one blade was split longitudinally and had separated from the hub. During the landing, the airplane struck trees.