The Party Cube is a giant white box found in Mario Party 4. In the game, Toad, Shy Guy, Boo, Koopa, and Goomba sent invitations to Mario and friends, allowing them to enter the Party Cube. Inside the Party Cube are a lot of game boards and minigames.
The Party Cube is rather mysterious, first appearing inside a floating bag over Princess Peach's Castle. Bystanders Mario, Luigi, Peach, Yoshi, Daisy, Donkey Kong, Wario, and Waluigi were initially frightened by the presence of this bag, which came in an almost intimidating aura including dark clouds, though it eventually burst to reveal the Party Cube. It was here that the characters were permitted to travel into the Party Cube and the events of Mario Party 4 began.
During Mario Party 4's ending credits, the story of how the game's hosts found the Party Cube is slowly revealed through a nostalgic-looking slideshow. Apparently, they came across it as a big, blank block trapped in a Warp Pipe, later carving pictures of themselves in it with unknown intentions. This led to a series of events that slowly morphed them into their Mario Party 4 forms, often through other characters appearing and rewarding them with their thematic costumes Mario Party 4 players are familiar with. A Lakitu also gave them the invitations seen in the game's opening while they were working on the Party Cube, encouraging them to put it to its use. The Party Cube was last seen escaping into the stars, being chased by the hosts.