Melt butter in a large saucepan; stir in flour until smooth. Cook, stirring, over low heat, about 3 minutes. Gradually whisk in hot milk until smooth. Cook, stirring constantly, until mixture boils; cook, stirring frequently, over low heat, until thick and smooth, about 10 minutes. Season with mustard, salt and red pepper.
Meanwhile, cook the macaroni in plenty of boiling salted water until al dente, or firm to the bite, about 8 minutes. Drain.
Combine the sauce, 3 cups of the cheese, and macaroni. Transfer to a buttered 9x13 baking dish.
Sprinkle top with remaining 1 cup cheese and then with buttered bread crumbs. Bake at 350º until browned on top and bubbly, about 45 minutes.
Buttered Bread Crumbs: Mix one cup bread crumbs with 3 tablespoons melted butter.
Notes
I like to make a double-batch and freeze one to bake later.