When was that a rumor, besides old school MH fanboys desperately wanting it to return to PS4? (I wouldn't care if it were true as I do have a PS4, but I highly doubt it has any truth behind it)
Considering Monster Hunter excels on handhelds, they'd be silly to publish it on PS4 at this point when the Switch is the ideal system for the series and makes it considerably more marketable to both Japan (which prefers handhelds) and the west (which prefers home consoles).
If it's NOT a localization of MHXX, then the only other thing it could be is a western release of Monster Hunter Online... which I really doubt we're getting.
"Monster Hunter World" Seems like a great follow-up to Generations though, so I'm betting on it being localized MHXX.
Edit: I wasn't taking into the spin-off Frontier series into this post, which is certainly a viable possibility but I don't know why they'd need to change the name for a western release unless there were some sort of issues.