4 million+? I don't think so. Splatoon 1 sold about 1.5, on the Wii U granted, but that's still a far cry from 4 and definitely 5 mil. The thing is, I don't think that the Wii U's install base held Splatoon back all that much. To back this up, Mario Kart 8 sold about 1.3 to 7's 2.8 and Smash Wii U sold nearly 0.9 to the 3DS's 2.5. As we can see, MK sold about 47% on Wii U as it did on 3DS and Smash did about 36%. This is in spite of the Wii U only having 14% of the install base.
Now, let's say that the Switch ends with about 18 million LTD in Japan, nearly 6x the Wii U. You may say that this means Spatoon 2 can sell 3x as much, but taking in to account, the Wii U's absurdly high attach-rate and the fact that there is not a direct correlation between install base and game sales, even 3 million, by doubling already massive sales, seems too optimistic.
Splatoon 2 wont be held back by userbase. It can literally have potential to hit the sky in sales.
-early life title (evergreen title)
-Its the next AC/Pokemon/MH/DQ
Your entitled to your opinion, it could do 2mil like you said, but if there is one game thats going to storm the charts, its going to be splat2. Just watch my friend. By years end, splatoon2 will be beyond 2mil easily. All depends on how many switches are out in the wild.
If switch is around 4mil this year, i bet splat2 will be around 2.5mil with years of legs :o