Initial sales for Windows 8 are higher than Windows 7 -- Microsoft has already sold 4 million upgrades for its operating system to consumers, and has sold tens of millions of copies to corporate customers.
Microsoft shared those numbers at a Tuesday event meant to entice developers to create apps and games for its platform. Just four days after Windows 8's launch, Windows Store has a sparse selection of apps (around 5,000 available in the U.S.), and the company is looking to grow the marketplace's catalog fast.
While Microsoft didn't specify how much better this upgrade's sales are performing against the last OS release in 2009, Windows 8 selling better than OS X Mountain Lion so far (though Windows' total install base dwarfs OS X's). Apple's OS update from June sold 3 million units in its first four days of availability.
Microsoft CEO Steve Ballmer told developers at the company's Build 2012 conference keynote today that by this time next year, there will be 400 million devices -- including PCs, tablets, and smartphones -- with Windows 8 installed. For comparison, Windows 7 sold over 240 million copies in its first year, and so far has 670 million installations.