Melt 4 tablespoons butter in a large saucepan. Stir in flour until smooth. Gradually add milk. Bring to a boil; cook, stirring, for 2 minutes. Reduce heat; add cheeses, mustard, salt and pepper. Stir until cheese is melted and sauce is smooth.
Drain macaroni, add to the cheese sauce and stir to coat. Transfer to a greased shallow baking dish. Melt the remaining butter, toss with bread crumbs and parsley. Sprinkle over macaroni.
Bake, uncovered, at 400 degrees for 15-20 minutes, until golden brown.