There's honestly no way to accurately predict right now.
EDIT: The best selling MH on 3DS sold approx 4m. Selling only 1m less than that across all platforms I think would be seen as pretty good.
Selling even less, though. I don't know.
My guess is that this will obviously be the best selling western Monster Hunter but the worst selling japan monster hunter. So my hack-kneed preditction is 2.5m physical units, just above the Wii version.