I think DS2 could get to 4M, it's not impossible. There isn't any competition during it's release time and didn't a recent study show this being one of the most anticapated games of the year? If this was coming out in November it would be screwed but I could see it posting big numbers quickly.
Everybody throws around Zelda comparison's, well Skyward Sword sold 3.42M copies in it's first month, was exclusive to one console, it required a controller not everyone had (Wiimotion+), and it released at a time when it's platform was dieing, with it's successor already announced. I know people will think it's crazy to compare DS2 to Zelda, (even though people do it all the time) but when you factor in the positives that it has right now and look at the things going against the last Zelda game I could see DS2 doing near first month SS numbers, and if it can sell 3M in it's first month, with a Wii U version coming later in the year as well it should get to 4M.
Skyward Sword sold to Zelda fans and Zelda is a much bigger brand. It has a built in audience that has been nurtured for decades. Just because it sold very well out of the gate doesn't mean a similar game will have repeat of it's success on another system. Who it's marketed to is still a factor and where those people are far more likely to be. It will have it's fans on the PS3/360 but I do think this is a good opportunity for THQ to really sell the Wii U version as there is a void wide open for them to fill if they play their cards right. If only Nintendo let them talk about that version.