The company established the target in August 2004, according to chief executive Jeremy Darroch, and has passed it with two months to spare.
Future targets would be internal, he said, including keeping customer defections below 10 per cent and increasing revenue per customer.
Sky recently launched its Anytime+ service, adding downloadable content to Sky subscribers who also have a Sky broadband connection, as well as the UK's first 3D channel.
By comparison, Virgin Media's subsciption TV service recently hit 3.76 million subscribers, while BT Vision's TV service has just over 500,000 customers.