Monster Hunter 4 seeks out Japan on September 14. Producer Ryozo Tsujimoto announced the release date in a Nintendo Direct today, during which he also unveiled two limited edition MH4 3DS XLs. Both designs are due at launch, and come with the game.
There's no news as yet on MH4 localization for the west.
Update: We've added in the new trailer that was shown during the Direct broadcast. You can find the MH4 3DS designs after the break, along with the undoubted highlight of today's Direct: Ace Attorney Iwata.