This 4 million number is equal to the number of individual consumers who bought a shrink-wrapped Windows 8 UPGRADE box. It was announced 4 days after the launch of the platform on the 26th.
http://www.amazon.com/Best-Sellers-Software-Operating-System...
[1]http://www.amazon.com/Microsoft-Windows-8-Pro-Upgrade/dp/B00...
[2]http://www.computerworld.com/s/article/9232961/Microsoft_sta...