En route from Washington to Norfolk at an altitude of 8000 feet in icing conditions, two engines failed. While descending to a lower altitude, the remaining two engines also failed and the propellers autofeathered. The crew tried to restart the engines and put the aircraft into a dive trying to drive the propellers out of the feathered position as they were not able to unfeather them normally. Eventually they managed to restart engine no. 4. Applying full power to this engine caused the Viscount to enter a circling descent until it crashed into trees with almost no forward velocity. PROBABLE CAUSE: “The accident was attributed to the delayed arming of the engine ice protection systems while flying in icy conditions, resulting in the loss of engine power and attendant electrical energy required to unfeather propellers and relight sufficient engines to maintain flight.”