Windows 8 is here, and while it’s not universally beloved, it does appear to be bringing a lot of cash into Microsoft's coffers. Microsoft CEO Steve Ballmer announced today at the company's BUILD conference that Microsoft has sold 4 million Windows 8 upgrades to consumers since the operating system went on sale Friday.

“The level of embrace from enthusiasts, from people who want to get out there, is very, very high,” Ballmer said today.

Ballmer said yesterday that Windows 8 is selling faster than Windows 7. Of course, this is based on just a few days, and Microsoft is offering better-than-usual upgrade pricing this time around. Upgrades from previous versions of Windows to Windows 8 Pro cost only $39.99 for a downloadable copy (or $69.99 for a boxed DVD) until January 31, 2013. Additionally, people who bought Windows 7 PCs since June 2 can upgrade to Windows 8 Pro for only $14.99. By contrast, Windows 7 promotional pricing in 2009 was $49 for an upgrade to Home Premium and $99 for an upgrade to Professional.

Ballmer also had a big number to report on sales to business customers, apparently referring to volume licensing and Software Assurance subscription sales. “We have sold tens of millions of units to our corporate customers, who can upgrade when they want to, but have no time pressure to do that any time soon,” Ballmer said. Those who bought Windows 8 consumer upgrade licenses are also not obligated to install the new bits right away.

Microsoft had some good news on the app front today, too. The Windows Store—the app store for Windows 8—is sparse so far, but Microsoft said applications from the likes of Twitter, Dropbox, and ESPN are on the way. Additionally, PayPal will integrate with Windows Store apps.

Recent big-name additions to the Windows Store include Skype, Netflix, and Angry Birds Space. But there are still notable missing names, such as Facebook. The Windows Store had more than 9,000 applications available at retail launch.