The fantastic remake of Resident Evil 4 only needed 2 days to sell more than 3 million copies, and it seems to be on track to become the best-selling game in the franchise.
Because Capcom reveals that Resident Evil 4 has surpassed 4 million copies sold now, which by my estimates makes it the second-fastest selling game in the series. It might even be the fastest, as Resident Evil 6 shipped 4.5 million units earlier, but the sales slowed down a lot faster than RE4's will because the former didn't exactly get as great of a reception. The fact that Resident Evil 4's free The Mercenaries mode has become available today will definitely help as well.