Preheat the oven to 350 degrees F.
Soak the mushrooms in boiling water to cover; let stand until softened, about 15 minutes. Drain, chop and set aside. Meanwhile, cook the macaroni according to package instructions.
Melt 3 tablespoons of the butter in a large saucepan over medium heat. Stir in the flour, salt and cayenne; cook and stir for 1 minute. Add the milk; bring to a boil over high heat. Reduce the heat; simmer for 2 minutes, stirring occasionally. Remove from the heat; stir in the cheese until melted. Stir in the drained macaroni and the reserved mushrooms.
Pour into a 2-quart baking dish coated with cooking spray. Melt the remaining 1 tablespoon butter in a small bowl; add the breadcrumbs and toss well. Sprinkle over the macaroni mixture.
Bake until hot and bubbly and the crumbs are golden brown, 25 to 30 minutes.