I think it would be nice to have it on both platforms.
I recently bought a 3DS just for 3U and MH4. But the more gameplay I see, the more I realize how hard it will be to control on the 3DS.

This is not happening though.
No matter how much you wish it will.
With the Vita's low sales; Sony is very financially frugal and will not risk the development costs to put MH4 on a failing console.
I love the Vita's hardware and capabilities, but it will need a lot of help to come back from the dead.