Portal 2, which celebrated its one-year anniversary last month, has sold over four million copies worldwide.
Valve team member David Sawyer confirmed the figure in an interview with NeoGamr. This number includes both console and PC sales.
Today Valve released the Perpetual Testing Initiative level editor on Steam. Both PC and Mac players can download the DLC for free. The company declined to comment on whether the pack would be made available to owners of the PS3 or Xbox 360 versions.
The Portal franchise has pushed more than eight million total units to date.
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