Portal 2 has sold over 4 million units since its launch, Valve has revealed.
The company shared the figure for their first-person puzzler sequel during an interview with NeoGamr regarding the game's map editor DLC. Speaking of which, that very same DLC has been made freely available for owners of the game.
Entitled the Perpetual Testing Intuitive, the post-release update brings a simple but powerful level editor that lets players create puzzles how they see fit.
Valve boasts that while the editor is accessible for new players, it's robust enough to satisfy the more hardcore.