I think that unless MH3G or MH4 gets better numbers in 3DS than MHP did in PSP, MH4 will be ported to Vita.
Because as of now the Japanese MH fan base is on PSP, not in 3DS.
Transitioning the series to new hardware - which we would all agree had to happen at some point - is not the same thing. It wasn't exactly coincidence that 3G and 4 were announced almost simultaneously - Capcom wanted to send a clear signal that 3DS was the series' primary platform from there on out.
MH3G was a rehash of a rehash which fans already played once or twice. MH4 was the new game.
It would be like if Capcom announces SSFIV Turbo and SFV for WiiU. The first one is a rehash, and at least the new one would be very likely to end in the others too.
No transition was needed for any IP that moved to other console. It's just Capcom being Capcom, they like to release as much ports and rehashed / refurbished games in as much as places as possible.
Obviously they need to move to a new generation, and this time they choosed to release first in one device instead as they use to do since they started with their multiplatform approach some years ago.
This is a business for them, not a religion. They will get as much as money as they can in any device they can. MH was released on PSP and not in DS because PSP was capable to move it and DS not. Now Nintendo has a device which can move it, so they got MH too. But it wouldn't make sense at all from Capcom side to don't release MH on Vita.