Bowser's Gnarly Party

"Welcome to Bowser's Gnarly Party! I'm a Koopa Kid. It's my board. I do what I want! 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 are everywhere, and are 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 he 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. If the player loses, half of their coins are taken away and Bowser 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 the British English localization). 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 they choose; instead, that box becomes locked. After eliminating two 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 more coins the player will be awarded as well.

In Story Mode, if the player beats their opponents on this board, Bowser challenges them to The Final Battle!; once beaten, the player completes Story Mode. Additionally, this board will be unlocked for play in Party Mode. It is not necessary to beat The Final Battle! to unlock the board.