The fourth floor is a supposedly hidden floor of Princess Peach's Castle. While most copies of Super Mario 64 depict the castle with only three floors alongside a basement, some rare cartridges include a floor above the third.
The contents of this floor along with the way the player accesses the floor vary from copy to copy, although most depictions of the floor describe it as a room with various branching halls that could have Boos roaming the area.
The fourth floor has no set appearance as far as people are known. Many players who have experienced the fourth floor has spoke of wildly different areas. This is likely a result of the shifting changes with the castle's layout, as a result of the Internal Plexus theory.
The most common depiction of the fourth floor is that it has various branching hallways where Boos will roam. This version is illustrated by SK-intheballs, where the textures retain the brown bricks and red carpet of certain rooms of the castle. Alternatively, some presume that the walls of the fourth floor would take on a more space-like look, as the walls of the castle were progressively painted from the hills on the first floor to the more starry look of the third floor.
Another rendition of the fourth floor is by Youtuber Greenio. In Greenio's Lost Tapes, the final tape is set in the fourth floor, which appears much differently to reports. In this rendition the floor is separated into two sections; the Hallway and the Labyrinth.
The Hallway is tiled in blue, orange and red bricks with multiple hallways branching out from the main hall. Boos are present here. Many pillars are visible down the hallway which leads to a balcony and The Toad Shrine. In one of the many rooms, there is a pipe leading to the labyrinth. The Labyrinth is a giant maze filled with Boos and Whomps. Hidden behind a wall is the Core, the representation of the A.I.
The Toad Shrine
- Main article: The Toad Shrine
A known area within the fourth floor is the Toad Shrine, a large metallic statue of Toad. The importance of the Toad Shrine is unknown, but it has appeared in many of the people's renditions of the fourth floor.
Some photographs of supposedly early designs for the fourth floor have risen to the internet during the recent conspiracy craze. one of these photographs is of a early design, relating to the 1995/07/29 build. It appears to be a scrapped concept, and it is written in Japanese, leading to the conclusion this was developed by a design team in Nintendo of Japan's offices or subsidiaries. This document has no formal translation yet on the wiki, but an editor contacted a Japanese friend who read this, confirming that it does indeed show a scrapped 4th floor for the castle.
Assuming that the existence of the fourth floor relates to the Internal Plexus theory, it's likely that the fourth floor is the darkest crevasse of the dynamic A.I.. With the fourth floor being so hidden and, following the idea that it is an unlockable bonus floor for experienced players, only accessible to those the A.I. deems as worthy and skilled enough. This would also explain why the floor keeps changing for certain players, as the A.I.'s dark intentions change depending on the player it is personalizing the game for.