During a keynote at the BUILD developer conference, Microsoft CEO Steve Ballmer announced that the company has sold 4 million individual upgrade licenses of Windows 8 in the four days that have passed since general availability last Friday. Emphasis on individual; this figure doesn’t include the corporate accounts which amount to tens of millions of additional users of the OS.

During the event, Microsoft showed off a number of different devices — on top of their own tablet, the Surface — to really promote the Windows 8 ecosystem. Both tablets and laptops built for Windows 8 were showcased, some of the latter coming in a convertible form factor, and/or offering a touchscreen. Microsoft also proudly announced that there were over 9,000 apps in the Windows Store by general availability.

It isn’t just about quantity though; they also celebrated quality apps on the platform, such as Skype, Netflix, and Angry Birds Space, while anticipating even more apps headed to Windows 8, such as Dropbox.