Black Sabbath originally had four members: Ozzy Osbourne, Tony Iommi, Geezer Butler, and Bill Ward. The quartet formed the core of the band that helped establish heavy metal in the early 1970s.
Osbourne sang, Iommi played guitar, Butler played bass, and Ward played drums. Before adopting the name Black Sabbath, the musicians performed together in Birmingham, England, under earlier names including Polka Tulk Blues Band and Earth.
The number four refers to the original founding lineup, not every musician who later played with the band. Black Sabbath experienced numerous changes over the decades, with singers such as Ronnie James Dio, Ian Gillan, and Tony Martin, plus drummers including Vinny Appice and Cozy Powell. Those later additions do not alter the original-member count.