Place potatoes on baking sheet. Bake at 350 degrees F for 1 hour and 20 minutes. While baking, combine the crab, mayonnaise, Old Bay seasoning, green onions, butter, and 4 oz of the cheddar cheese in a large mixing bowl and mix until well blended. When potatoes are done, let stand for 15 minutes. Slice potatoes into lengthwise halves. Scoop potato flesh into the mayonnaise mixture until a large hollow cavity has been created within the potato. When all the potatoes have been scooped, mix the filling mixture one more time until well blended. Fill potatoes with mixture. Top potatoes with remaining cheddar cheese. Bake potatoes for an additional 15 minutes at 350 degrees F. Let stand for 10 minutes. Top with sour cream as desired when done.