Valve has revealed in an interview with NeoGamr that the game of the year award winner and currently the most confusing game on the market Portal 2 has sold over four million units worldwide across PC, Mac and console. Its predecessor also sold over four million copies, giving the series as a whole a pretty impressive sales record.
Also of note is the release of the new (and free) Perpetual Testing Initiative DLC for both PC and Mac. This adds a level editor for building and uploading your own twisted creations. No doubt that this will give a nice boost to the already healthy sales figures.