Let's not forget:
3U for Wii U was released - I'm assuming - primarily because the 3DS lacked online play.
What's the incentive or value piece that Capcom can market a 4U on Wii U? I'm sure everyone here (including myself) would all hold "1080p" as a selling point but that's really something only a small percentage of potential buyers could really care about.
I'd venture to guess if you're a Wii U owner, you already own a 3DS. So, again, looking from a Capcom perspective of "how do we move Wii U units" - what's are the selling points?
Additionally - I'm wondering how online play would work between someone on Wii U versus someone on 3DS. 3DS doesn't have voice chat, while 3U on Wii U has.
So in just doing some thinking, I'm almost with Capcom here as to why they wouldn't release a 4U on Wii U. But of course, me being a huge Monster Hunter fan, would totally love a release on it.