Rock Milestones: Led Zeppelin’s IV zeroes in on Zeppelin’s fourth LP, an untitled effort that shot to the top of the charts upon release circa November 8, 1971, and single-handedly shaped the landscape of hard rock over the ensuing decade. Rock Milestones: Led Zeppelin’s IV goes behind the scenes to investigate why — and how — this album became a decades-long phenomenon and the inestimable impact it made on American music and pop culture in particular.