Add the flour and cook, whisking constantly, until the paste bubbles a bit, about 2 minutes (make sure it doesn't brown). Continuing to whisk, add 3 cups of the hot milk and cook until the sauce thickens, about 5 minutes. Season with salt and pepper. Bring the sauce to a boil, reduce the heat to low and cook for 2 to 3 more minutes. Stir in the dry mustard, and then gradually add the Cheddar and Muenster cheeses, stirring constantly until all of the cheese has melted into the sauce. Stir in the hot sauce and the remaining 1 cup milk. Add the cooked pasta to the sauce and stir to coat. Stir in the crispy bacon and season with salt and pepper. You'll need 2 cups of mac and cheese for the sandwiches.
For the grilled cheese: Top all 8 slices of bread with a slice of American cheese. This will act as your glue. Spoon 1/2 cup of the mac and cheese onto four of the slices, sprinkle with some basil, and then add a dollop of the spicy peppers. Top with 1 or 2 tomato slices, and cover with a second slice of bread.
In a large nonstick skillet set over medium heat, spread 1/3 cup of the grated Parmesan in an even circle roughly the same size as your bread. Let the Parmesan begin to melt and brown, 2 to 3 minutes. As soon as it gets golden brown, set a sandwich on top of the Parmesan. Cook for 2 minutes more. The Parmesan will adhere to the bread and create a nice crispy crust. Remove the sandwich from the skillet and repeat the same process for the other side of the sandwich. Depending on the size of your pan, you may be able to do two sandwiches at a time. If needed, keep finished sandwiches warm in a 200 degree F oven while the others are cooking.