World 4 (Super Mario Run)

World 4 is the fourth world in Super Mario Run. Although its exact terrain is unknown, it contains four levels that consist of the fortress, grassland, underground, and airship themes, respectively. This world is available after completion of World 3, and unlocks World 5 upon its own completion. 
==Levels==
=== Cutting-Edge Spire ===
A fortress level with Grinders.
=== Slope to Success ===
A grassland level with slopes (some steeper than others), Goombas, and Koopa Troopas.
=== Danger High and Low ===
An underground level with Buzzy Beetles and Munchers.
=== Firing the Airship's Burners ===
An airship level containing Burners and Ninjis, as well as the game's fourth boss battle, against Boom Boom.
==Enemies introduced==
*Buzzy Beetle
*Burner
== Obstacles introduced ==
* Grinder