Excellent milestone. I don't think it'll catch MGS4 though. It depends on how much it'll be bundled. I think MGS4 will end at about 5.6million and Uncharted 2 at 5.4million
Next 4mill milestone will be crossed by LittleBigPlanet, God of War 3, and Gran Tursimo 5. Then LittleBigPanet 2, Killzone 3, inFamous 2, and Twisted Metal which will all be big.
EDIT: I think it can catch MGS4 looking at the legs of Uncharted 1!