Personally, I believe that due to the Wii U's current sales predicament, Splatoon has a chance at reaching 2.5-3 million lifetime provided that it keeps the momentum of Nintendo's hype/advertisement. If it is supported well like promised 2.5-3 million lifetime is quite likely and most certainly a success considering current Wii U sales.
The only way I can see it surpassing those sales is if the Wii U were to suddenly take off. While this is possible, there is a strong chance that it will not happen. I personally think come this holiday season if Nintendo made an advertisement for the Wii U that has two segments featuring exclusives, one featuring fun multiplayer experiences (Smash Bros, Mario Kart, Splatoon, Mario Party etc.) and the other segment featuring 'mature games' (Bayonetta, Devil's Third, Fatal Frame and Zombi U etc.) this could draw some attention to the console.
Without enough hype backing the Wii U itself, it is difficult to see Splatoon reaching 4 million.
At least we know that the first shipment in Japan has completely sold out: http://www.gamnesia.com/news/splatoon-has-sold-out-in-japan-nintendo-apologizes-for-shortages#.VWkk3fmSxLl
This is a good indicator for Splatoon's success.