In large saucepan, over medium heat, melt 2 tablespoons margarine. Stir in flour until smooth. Gradually stir in milk; cook, stirring constantly, until mixture thickens and begins to boil. Remove from heat. Blend in cheese and mustard; stir in macaroni. Pour into lightly greased 1-1/2-quart baking dish; sprinkle with crumbs. Melt remaining 2 tablespoons margarine; drizzle over crumbs. Bake at 400 degrees F. for 30 to 40 minutes until bubbly and browned.