Monsters, Inc. was Pixar’s fourth feature-length animated film released theatrically.
Pixar’s first four theatrical features were Toy Story in 1995, A Bug’s Life in 1998, Toy Story 2 in 1999, and Monsters, Inc. in 2001. This places Monsters, Inc. directly after Toy Story 2 in the studio’s early feature history.
The film was an important step in Pixar’s development because it introduced a new original world after two Toy Story films and A Bug’s Life. Its industrial monster society, closet-door system, and fur-heavy character animation expanded the studio’s technical and storytelling ambitions.
The number refers to Pixar’s feature-length theatrical releases, not every short film, television project, or production involving Disney. It is also not the fourth film chronologically set within Pixar’s fictional universe. Those distinctions explain why “fourth” is the precise answer to this filmography question.