Irrational Studios adventure shooter BioShock Infinite has sold over 4 million copies.
That's according to Take-Two CEO Strauss Zelnick, who offered the figure during a financial results call today.
Zelnick also said the FPS is the highest-selling game in US this year, citing NPD figures (which only track sales of physical retail titles in North America).
As revealed earlier today, Irrational Games is developing BioShock: Infinite DLC that will bring players back to the underwater city of Rapture.
Coinciding with the announcement that it has today released the first Infinite DLC - a challenge pack called 'Clash in the Clouds' - Irrational also revealed 'Burial at Sea', an upcoming two-part campaign add-on featuring a new story for the BioShock Infinite protagonists Booker and Elizabeth. Hit the above link for more details and a trailer.