I don't think this will happen, though if it did I would be really happy. I purchased a Vita and a 3DS so I don't really mind where MH4 is, at this point I just have to pray they release it in the west. Though it would be interesting if Capcom suddenly had a change of heart with Nintendo. It seems like after MH3U for Wii U and MH3 coming to 3DS in the west, that Capcom was going all in with Nintendo platforms. Perhaps some of the lack luster Wii U sales are causing them to consider putting MH4 on different platforms? Who knows, but I still have a hard time believing they would put it out on Vita the same day.
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