How Long Cook 1 Lb Meatloaf. How long does it take to cook a 4 lb meatloaf at 400 degrees? A 3 lb meatloaf will take about 1 hour to 1 hour. how long to cook 3 lb meatloaf from recipeler.com This gentler cooking minimizes moisture loss as the meatloaf cooks, yielding a moister and …