While pasta is cooking, pour 4 cups milk into a large glass measure or other microwaveable container. Heat until hot, about 3 minutes on high. Or, heat in small saucepan on stove, just until scalded. Grate your cheeses but remember to keep 8 ounces separate. This won't be added to the roux.
While milk is heating, melt ½ cup butter in a large saute pan over medium-low heat. Once butter is melted add ⅓ cup flour. Stirring constantly, cook until flour flavor is cooked out, about 1 ½ minutes. If it starts to boil too much, reduce heat to low.
Add most of the milk, about 3 cups, not too slowly as it will get clumpy if you add too slowly. You'll know when to speed it up. As you pour in the milk, stir with a whisk to help smooth out any lumps. Once milk is incorporated, turn heat back to medium and stir until sauce starts to thicken, approximately 5 minutes.
Add 1 teaspoon salt, ½ teaspoon cayenne pepper, and ¼ teaspoon garlic powder. Be careful not to add more salt than indicated until after all the cheese has been added. At that time you can taste and add more if desired. The cheese has salt so if you're not careful, it can become over salted.
Turn heat to low and add grated cheeses (except for the 8 ounces you set aside). Stir until all the cheese is melted. Taste and add more salt if desired. If cheese sauce seems too thick for your liking, add more, up to a cup of milk, stirring to incorporate and to maintain smooth texture, I almost always do this. Turn off the heat.
Set up your casserole dish next to the cheese sauce. Pasta is probably stuck together now. Just splash the pasta very briefly with cold water and toss with your hands to separate.
Pour just enough cheese sauce into the bottom of your casserole dish to cover. Add a layer of pasta to cover the bottom, not too much. You want the layer to be thin enough so the sauce can seep down- a bit more than a single layer. Sprinkle some grated cheddar cheese in a thin layer over pasta. Pour enough sauce over this layer to cover completely, almost half of the sauce in your pan.
Repeat with more pasta, remainder of cheese sauce and another thin layer of cheddar. Cover with aluminum foil.
Bake, covered for 30 minutes. Meanwhile, place 1 sleeve of saltine crackers in a gallon-size plastic bag and crush with your hands or a rolling pin. You want most of the pieces to be about dime sized. Add 3 Tablespoons melted butter and toss to incorporate fully into the crackers. Sprinkle with a dash of cayenne pepper and/or red pepper flakes. Add a dash of black pepper for even more of a bite if desired.
After the casserole has baked covered for 30 minutes, remove foil and sprinkle the crumb mixture over the top, covering completely. Place back in the oven and bake uncovered until bubbly at the edges, approximately 15-20 minutes. Crust should start to turn golden.
Remove from oven and serve hot.