Portal 2 has sold 4 million copies since its release in April 2011, Valve's David Sawyer told NeoGamr in an interview published today. Sales of that caliber put the sequel roughly on par with the original Portal, which sold nearly 4 million copies, not including Steam units.
It wasn't made clear if this tally represents sales across all platforms, and Valve has not responded to GameSpot's request for comment as of press time.
Available today for PC copies of Portal 2 is the Perpetual Testing Initiative level editor. The tool allows players to create, share, and download Portal 2 levels. Players can publish their creations to the Steam Workshop, where users can browse, vote on, and select other levels to try out.
The Perpetual Testing Initiative DLC has not been announced for the Xbox 360 and PlayStation 3 versions of Portal 2. For more on the title, check out GameSpot's review of Portal 2.