Metallica had released four studio albums before The Black Album, making the 1991 record the band's fifth studio album.
Those earlier albums were Kill 'Em All (1983), Ride the Lightning (1984), Master of Puppets (1986), and ...And Justice for All (1988). Together, they established Metallica as one of the leading thrash-metal bands of the 1980s.
The Black Album changed the band's presentation without abandoning heavy music. Its songs generally used slower tempos, clearer production, and more pronounced hooks than much of the earlier material. That stylistic shift helped the album reach listeners well beyond the band's established metal audience.
A common mix-up is counting live releases, compilations, or demos as studio albums. The number four refers only to Metallica's completed studio albums released before the self-titled 1991 album.