Raymond is building a new dog run with a raised border in his backyard. The boards on the two shorter sides are 6 feet thick, and the boards on the two longer sides are 4 feet thick. Raymond wants the outer length of the dog run to be 4 times its height and the outer width to be 2 times its height. He also wants the boards to rise 4 feet above the level of the grass in the dog run. Use x as the variable throughout the entire problem. Write a simplified expression in standard form that represents the volume of the grass in the dog run.