What did Monster Hunter 4 add?

Height. It gave the hunt a third dimension, two new weapons, and a monster that evolves mid-story.

Monster Hunter 4 launched in Japan on September 14, 2013 for the 3DS and rebuilt the hunt around verticality: climbable walls, ledges to leap from, and mounting, the rodeo attack that drops a monster for the whole party. Underwater combat was retired, and the freed budget bought the two most technical weapons in series history, the Insect Glaive and the Charge Blade, completing the modern fourteen.

Its story engine was the Frenzy Virus, a plague that drove monsters berserk, centred on the flagship Gore Magala, a blind, feelers-first wyvern that matures into the radiant Shagaru Magala. The original 4 was never localised; the West began with the expanded 4G (October 2014 in Japan) as 4 Ultimate on February 13, 2015. Between them the pair sold more than 8 million copies.