In saucepan over low heat, mix flour and butter together until blended, making a roux. Cook about 3 minutes. Blend in chicken stock and milk. Continue to cook on low about 6 minutes. When sauce begins to thicken, add garlic, mustard, sea salt, pepper, colby and Parmesan cheeses, and add 3 cups of cheddar cheese (reserve 1 1/2 cups cheddar for topping). Cook about 5 more minutes on low. Set sauce aside while macaroni cooks.
In a pot over high heat, bring water to a boil, and add salt. After salt dissolves, add macaroni. Cook about 8 minutes, stirring occasionally. Drain macaroni noodles, and add them to the sauce.
Place mixture in buttered casserole dish, and top with 1 1/2 cups shredded cheddar cheese. Bake at 375 F about 30 minutes or until it reaches an internal temperature of 165°F.