Preheat oven to 375 degrees
.Cook macaroni in 5 quarts boiling water for 8 minutes.
Drain macaroni.In medium saucepan, melt butter.
Stir in flour, mustard, and salt.Gradually stir in milk.Cook and stir until mixture thickens slightly.Remove from heat.
Add 2 cups cheese to milk mixture and stir until melted.Stir in macaroni.Turn into greased 2 quart baking dish.
Top with remaining 2 cups cheese.Bake 30 to 35 minutes or until bubbly.