Nintendo announced today that more than four million Nintendo 3DS systems have been sold in the United States. According to Nintendo, the system has sold more in its first nine months than Wii did. The milestone comes as a result of strong sales for Super Mario 3D Land and Mario Kart 7, which have become the first games to sell over one million units in the United States. This follows Ocarina of Time 3D, which crossed the one million milestone worldwide last year...