MH4 will sell great on 3DS.
And even if it will not beat numbers of 3rd in the end, every major series has ups and downs. For example, even if "Final Fantasy fanbase established on the PS Platforms", some titles historically selling worser than famous predecessors. Same with Pokemon on Nintendo platforms, etc etc... And sometimes new games selling better than previous. Like Dragon Quest IX.
Monster Hunter 3 Ultimate sold 1,7 million on 3DS (full price) and more than 300K (best price) in Japan - 2+ million japanese users bought another version of 3 on 3DS. I don't see a problem for completely new Monster Hunter 4 to sell 4-5 million lifetime.