Preheat oven to 400 degrees F. Melt one tablespoon of butter in a small bowl. Mix in the bread crumbs, and set aside.
In a large pot of water, pour in 1/2 tablespoon of salt and boil the macaroni until tender. About 11 minutes. Drain water. Leave macaroni in the strainer. Set aside.
Use the same large pot, and carefully wipe it dry (it will be hot!). Add the remaining 3 tablespoons of butter, and melt over medium heat.
Stir in the garlic and ground mustard. Cook until fragrant. About 30 seconds or so. Stir in the flour and cook until golden, about a minute. Whisk in milk and chicken broth. Bring to a simmer, and continue to cook, whisking often. Cook until mixture has thickened, about 6 minutes.
Remove from heat and add in all the cheese. Stir until cheese has completely melted. Season with salt and pepper to taste. Stir in the macaroni until well combined.
Pour into a greased 8x8 baking dish. Sprinkle the bread crumbs evenly over the top. Bake for 15 to 20 minutes, or until hot and bubbly.