As a whole new number, Monster Hunter 4 will feature an entirely new story and base of operations from the previous games. Unlike the previous games in which the player started as a hunter for hire who went on hunting quests from a single stationary base of operations, in Monster Hunter 4, the player is hired as a member of a roaming caravan, traveling the world. The creators have placed a greater emphasis on "story" and "adventure" than in previous games, adding more NPCs than before, as well as multiple unique villages that can all be used as bases of operations.
For each new game in the series, new stages and action mechanics have been added. Monster Hunter 4, which was developed alongside Monster Hunter 3, will introduce the concept of elevation and slopes, an upgrade to the grab mechanic incorporated from Dragon's Dogma that allows players for grab and attack while hanging on to ledges and monsters, and the addition of new "ruins fields" stages to explore and hunt in. Despite the parallel development with Monster Hunter 3, there will be no underwater stages this time around.
As with every incarnation of Monster Hunter there will be new monsters and subsequent new equipment and most likely a new upgraded "G" version of the game released within a year that adds even more content at the full retail price. Sadly, Capcom has not revealed any intention to release the game on the PS Vita, as some Japanese players out there were desperately hoping for, and as far as the Monster Hunter series is concerned, remains hand in hand with Nintendo. (&$%#!!!)
Monster Hunter 4 is scheduled for release in Spring of 2013 (if the Mayans don't have their way) on the 3DS.
CAPCOM : モンスターハンター４公式サイト [CAPCOM : モンスターハンター４公式サイト]