In large pot melt the stick of butter. Add flour and stir to create a roux. Stir over medium heat for one minute to let flour and butter settle together.
Slowly add about 1 cup of milk at a time. Stir in milk and turn up heat to med-high, just until sauce is thick and bubbly.
Remove from heat and stir in 3 cups of shredded cheese. Once cheese is incorporated, now it's time for your secret weapon!!
Remove from heat and add the sour cream. Season with salt & pepper to taste. Stir in cooked macaroni.
Pour mixture into a greased 9x13 pan and top with remaining 1 cup of shredded cheese.
Bake at 350˚F for about 15 minutes or until cheese has melted on top.