Use 2 tablespoons of the butter to generously grease a 13 by 9-inch baking pan all over the bottom and sides.
Over medium-high heat, bring a large heavy pot of well-salted water—you want the water to be salty like the sea—to a boil.
Add the macaroni, lower the heat to a simmer, and cook according to the package directions for al dente.
When the pasta is finished cooking, drain it in a colander, run it under cold water to stop it from cooking any further, and set it aside.
In a large saucepan over medium heat, melt the remaining 2 tablespoons of butter.
When the butter is just melted, sprinkle the flour into the pan and cook, whisking the flour into the butter, for about 1 minute, or until the butter and flour form a blond paste.
Slowly and little by little, whisk in the milk, taking plenty of time to whisk between additions of milk, until all the milk is whisked into the paste and a smooth sauce remains.
While whisking, bring the mixture up to a simmer so that it’s just beginning to bubble, then remove the saucepan from the heat and whisk in 2 cups of the shredded cheese.
When all the cheese has melted into the sauce, season with salt and pepper to taste.
Preheat the broiler.
Add the drained macaroni into the sauce and stir to combine.
Transfer the macaroni and cheese into the prepared pan.
Scatter the remaining shredded cheese over the top of the mac ’n’ cheese.
Put the pan in the broiler and cook until the cheese is golden brown on top, checking frequently, 3 to 5 minutes.
Remove the pan from the broiler and let stand for 10 minutes before serving.
