The 4 layer is a flexible cable. Most SRM's use cable drive shafts that are heavier than those used in the curved shaft units.They are very durable and are better than solid steel for absorbing shock loads when using blades although solid is available with some models.
Echo makes very good engines, the newer tornado is more emission compliant. Catalytic converters are becoming more common but add cost and operating temps are higher but what do you do?