Preheat oven to 375°F and grease a 9x13-inch baking dish.
Cook pasta according to package directions until al dente, drain and set aside.
In a large pot, melt 4 tablespoons butter over medium heat, then whisk in flour and cook for 1 minute until golden.
Gradually whisk in milk and cook, stirring constantly, until mixture thickens, about 5 minutes.
Remove from heat and stir in cheddar, Gruyère, and fontina until melted and smooth.
Season sauce with salt, pepper, and garlic powder, then fold in cooked pasta until well coated.
Transfer mixture to prepared baking dish and spread evenly.
Combine panko, melted butter, and Parmesan, then sprinkle over the top.
Bake for 25-30 minutes until golden brown and bubbly, then let rest for 5 minutes before serving.