MH4 may not reach the highs of 3rd portable and if you see it as a disappointment in that perspective (even though thats a very weird way to see it since its still selling a shit load and will probably have a LTD close enough to 3rd) then okay.
However, it really was the best case scenario for Capcom. Low dev costs + placing it on the platform where it would perform the best. Vita was definitely a no go due to the size of the userbase at the time. Maybe MH could have accelerated it but Capcom had a better and a less risky alternative namely the 3DS.
One interesting thought is if putting MH4 on the PSP would of resulted in greater sales (alongside with the cheaper dev costs).
MH4 was announced before the Vita was even released but even then Capcom probably still would've gone 3DS.