This video shows you how to get the skull that is hidden in Bad Piggies Flight in the Night Bonus Level 4-II. The skull is located at the extreme right of this very long level. Make your contraption as thin as possible, using only the wheels, propeller, and engines to make your way across the level. When you arrive at the cave, just let the contraption naturally fall along the bumpy path until you reach the skull at the bottom.
If you have a different strategy for completing this level feel free to leave a comment below.