In 5-quart Dutch oven, melt 4 tablespoons butter over medium heat. Beat in flour, onion salt, garlic salt and pepper with whisk until thickened. Cook 1 minute longer. Beat in milk. Heat to boiling over high heat, stirring frequently; boil and stir 1 minute. Remove from heat. Stir in 3 cups of the cheese until melted. Stir in macaroni, chicken and frozen broccoli. Pour into baking dish. Sprinkle remaining 1 cup cheese on top.