Please explain how Capcom "needs" MH to grow in the West.
"Wants" to grow it? Sure, I'll accept that. Why wouldn't they want more sales? "Needs," though? Not so much.
After all, mainline MH games still sell 3-4 million in Japan alone, while spinoffs sell around 2 million (if not more). Are you saying this isn't enough?
Personally, I think it makes sense that Capcom would want to try putting a mainline MH game on PS4, especially given that system's Western sales, but as everyone sane has already stated, making such a game exclusive to PS4 basically would be the most stupid thing they could do ATM. (Such a game *may* sell more copies in Western markets than the 3DS MH games--although I don't think that's a given--but it's also sure to sell millions fewer copies in Japan.)
Multi-platform MH5 makes the most sense to me. Should that happen, though, I'm till not sure the PS4 version is going to sell as well as some here seem to think it will, either inside Japan or in other regions.
To me, there's a fairly limited audience for a game like MH outside of Japan (much like Dragon Quest), and simply making it prettier, etc., isn't going to cause that audience to expand. In Japan, meanwhile, most MH fans seem to love the series for its local multiplayer component, which means the bulk of the fanbase is likely to buy and play the Switch version rather than the PS4 version, assuming both are released there.
I guess what I'm saying is that the best-case scenario I can personally see for MH5 is that it's Switch-PS4 multi-platform, with the Switch version being the "main" one.