Honestly while I think 3D will definitely add to the gameplay. I can't see that defining feature being the reason for Monster Hunter 4 coming to 3DS. I'm guessing Nintendo used all the influence they had to convince Capcom to support 3DS. Also if Monster Hunter 3G doesn't sell that well and Capcom's investors have their way then we will never see MH4 come to 3DS. Instead it will likely get ported to PSVita or another platform!
I am far from convinced that MH4 will come to 3DS, I'm guessing it will be cancelled by mid next year. Its way to early for Capcom to be revealing it. Considering upon the announcement Capcom's shares dropped by over 8%, I think Capcom's share holders will do everything they can to prevent MH4 from coming to 3DS. Then if MH3G fails to sell butt loads the investors will leave Capcom with little choice but to cancel MH4!
-JC7
"In God We Trust - In Games We Play " - Joel Reimer