In a medium saucepan, melt the remaining 4 tablespoons butter over medium heat. Whisk in the flour and cook, stirring, for 2 minutes. Whisk in the milk gradually and cook, stirring, until the sauce is smooth and thickened, 5 to 7 minutes; season with salt and pepper. Pour the sauce over the casserole and spread evenly with a knife. Top with the potato chips and bake until golden-brown and bubbling, about 25 minutes.