Cut potatoes into quarters.
Cook potatoes for about 25 minutes in boiling salted water. You will know they are done when they pierce easily with a fork.
Drain potatoes and put them in a large mixing bowl.
Combine potatoes with sour cream, 4 Tbsp butter, salt and pepper.
Beat the potato mixture until light and fluffy on low speed.
Pile into a 9x13 casserole pan. Cover and refrigerate overnight (I have made mine the same day too).
Bake covered in a preheated 325 degree oven for an hour.
Toss with 2 Tbsp melted butter.
Continue baking uncovered about 30 minutes longer.