Preheat the oven to 375 degrees F. Grease a 9x13 inch baking dish.In a large saucepan, bring 4 cups of salted water to a boil over high heat. Add the pasta and cook, stirring occasionally, for 8 minutes. Stir in the milk and cream cheese and cook until the cream cheese has melted and the pasta is al dente, about 5 minutes more. Remove the pan from the heat and stir in 2 cups of the cheddar and 3 tablespoons of the butter. Season with salt, pepper, and cayenne. Stir in the spinach and artichokes. If the sauce feels too thick, add ¼ cup of milk or water to thin it.Transfer the mixture to the prepared baking dish. Top with the remaining 1 cup of cheddar.In a medium bowl, stir together the crackers, the remaining 3 tablespoons of butter, and the garlic powder. Sprinkle the crumbs evenly over the mac and cheese.Bake until the sauce is bubbling and the crumbs are golden, about 20 minutes. Let cool for 5 minutes and serve. Store any leftovers refrigerated in an airtight container for up to 3 days.