Monster Hunter 4

[Arjahn]

I've honestly only played Tri because screw the PSP, but as far as I know, you kill monsters then carve their corpses to get materials which you then use to get better gear. Think of the levels like dungeons in an MMO only a lot more open ended with multiple objectives and a lot more focus on the Boss fight.
In single player, there is no set XP system, all of your stats come from your items and equipment. The same goes for multiplayer, although there is a Hunter Rank system where you get points based on how well you did in your last hunt that scales like an XP bar. This doesn't really affect your stats or anything, but unlocks more missions to play through that scale to the type of gear you should have by than point: It's more there for balance than anything so a party of 3 really skilled hunters doesn't get stuck with one ch00b.