While is certanly difficult, it is possible, seeing that splatoon is doing good at the launch.And as Mario kart 8 and smash bros, Splatoon is a title that will(probably) be supported through all of Wii U cycle, getting new content and such.So i dare to guess that Splatoon sales will come mostly from its legs.And if it has even half of the monstruous legs that mario kart 8 has, it certanly can reach 4 million in a couple of years.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1