Gusteau’s restaurant originally held five stars.
In Ratatouille, Auguste Gusteau is introduced as a celebrated chef whose Paris restaurant reached the highest status in the film’s fictional rating system. The restaurant’s prestige is central to Remy’s dream: Remy admires Gusteau and wants to cook in the famous kitchen despite being a rat.
The rating changes after food critic Anton Ego publishes a devastating review. Gusteau’s restaurant loses one star and falls from five to four. After Gusteau’s death, the restaurant loses another, leaving it with three stars under Skinner’s management. That sequence is why viewers may remember several different numbers depending on which point in the story they have in mind.
The film’s five-star scale is also an intentional simplification. The Michelin Guide, which is associated with French fine dining, awards restaurants a maximum of three stars. Ratatouille uses five stars as a dramatic shorthand for Gusteau’s exceptional reputation, so the answer concerns the fictional restaurant’s original status, not the real-world Michelin system.