Nintendo says they expect... and they are always conservative on their approach.
4 million may be a bit high, but if Nintendo can produce that much and if demand is that high, just maybe the final result won`t be that far apart.
Take Wii for example, in it`s first month it sold past 3 million units (worlwide), from what i saw on the internet. 4 million for 3DS doesn`t seem that high.








