Bowser's Gnarly Party

"I built this board to celebrate Master Bowser's greatness! You'll love it, or I'll tell on you! Master Bowser is the best! If you don't see him now, you'll certainly see him later! Bwaha!"

- Koopa Kid

Bowser's Gnarly Party is a board located above a sea of lava in Mario Party 4. It is hosted by Koopa Kid. He built it in honor of Bowser's greatness and wanted the players to have fun on it – whether they wanted to or not. Here, Koopa Kids were everywhere, and were put in charge of the games and shops. In the very center of the board, a giant scorched Bowser Shell can be seen, surrounded by a building that resembles the Colosseum.

The main attractions of this board are the two stone bridges and Bowser. Every time a stone bridge is crossed, pieces of it will crumble off. When it is crossed three times, it collapses and a new one appears leading in the other direction.

Bowser is always on his platform somewhere on the board, waiting for a character to pass him (and changes his position every time a player trades 20 coins for a Star from Koopa Kid). He does different things to players depending on what size they are. When a player reaches him at normal size, he takes away half of their coins, or gives them fifty coins if they have none. If a player reaches him while miniaturized, he uses his fire breath to send them to a different starting point in the middle of the board. If a player gets to him in their mega form, he challenges them to either Bowser Wrestling or Panels of Doom (though the Bowser mega minigame is not The Final Battle! minigame if this is done on Story Mode). If the player loses, half of their coins are taken away and he still remains on the board. Victory results in Bowser leaving the board until a player trades 20 coins for the next star from Koopa Kid; he will then reappear and change his position. If a player reaches him while wearing the Bowser Suit, he gives the player 50 coins.

Every few turns, Bowser uses his magic to miniaturize everyone for that turn. This also disables the players' ability to use items, similarly to a MiniMega Hammer.

Landing on any of the Happening Spaces located behind the Gates will cause the Bowser head statue in front of the spaces to breathe fire, destroying one of the player's items.

The Item Minigame of the board is Koopa Kid Shuffle (Mini-Bowser Shuffle in PAL regions). A Koopa Kid puts an item in each of three boxes and then shuffles them around. After that, the player chooses one of them. However, the player does not get the item in the one he or she chooses; instead, that box instead becomes locked. After eliminating 2 boxes, Koopa Kid gives the item in the remaining box to the player. The Coin Minigame, Brake Bullet Bill, involves a Koopa Kid shooting a Bullet Bill at the player, and the player must mash to slow it down with their hands. The faster the player mashes, the shorter the speeding bullet will fly, and the player will be awarded more coins as well.

In Story Mode, if the player beats their opponents on this board, it will be unlocked for play in Party Mode. It is not necessary to defeat Bowser in The Final Battle!, and complete Story Mode.