The fourth Event slot always has a modifier active. These introduce dynamic new elements to the match. They rotate along with the map.
Meteor Shower and Energy Drink appear more frequently than Life Leech and Robo Boss. The latter two have only appeared a few times each since they were released.
Meteors rain down on players. They can fall anywhere on the map, and they deal 2000 damage to any Brawlers they hit while also destroying walls. Meteors also end any attack a Brawler is using. As the match goes on, meteors will begin to fall increasingly frequently. Avoid them and try to use them to your advantage by using them to trap enemy Brawlers.
You constantly lose little bits of HP. This does not reveal you in bushes. Hit/kill enemies or use a healing attack/Super/star power to heal. Killing enemies restores all your HP. Damaging enemies restores half the damage you dealt.
An energy drink occasionally spawns somewhere in the map. If you pick it up, you deal way more damage and move faster. This does not last very long so make the most of it. As the match goes on, Energy Drinks will begin to spawn increasingly frequently.
A Robo Rumble boss occasionally spawns and attacks Brawlers, walking toward the nearest one even if they are hiding in a bush. It does not have the rockets attack and its melee attack deals less damage. As the match goes on, Robo Bosses will begin to spawn increasingly frequently.
A large ring of green mushrooms spawns, creating an area that heals any players for 1000 health per second. It lasts for a couple of seconds before disappearing again. As the match goes on, Healing Mushrooms will begin to spawn increasingly frequently.