David Sawyer of Valve reported to NeoGamr that Portal 2 has sold 4 million copies since its release in April.  That puts the game on par with its predecessor, Portal, which sold nearly 4 million copies, not including Steam units.

However, Valve has declined to comment on whether or not the sales account if for all the platforms it was released on.  Would that be a bubble buster?

In other news regarding Portal 2, released today is the Perpetual Testing Initiative.  This tool allows player to create, share and download Portal 2 levels.  Players can also publish their creations in the Steam Workshop where other users can browse, vote on and select other levels to try out.

The Perpetual Testing Initiative has not been announced for the xbox360 and PlayStation 3 versions of Portal 2.

Source: GameSpot