User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-5460283-20130928175221/@comment-11685710-20130928185053

I think the devs already confirmed, that the region of MH4 is even farther away from the old world than Moga, Loc Lac, Yukumo and Tanzia. So I would guess the Rathians (and other monsters) of MH4 are a slightly different/differently adapted species.

That'd also explain the newly discovered (in 3rd gen) Brachidios in low rank quests: The stronger/older individuals migrated to the region around Moga/3rd gen's Volcano and Tundra and because of that only more experienced hunters are allowed to hunt them there, while the ones in MH4 are maybe even juveniles, that are weak enough to be hunted by (relatively) unexperienced hunters.

And of course the completely new monsters, that aren't found anywhere in the old world.