Number 29! Led Zeppelin - [untitled] (1971, Atlantic) Personnel: Robert Plant – vocals, harmonica Jimmy Page – electric and acoustic guitars, mandolin John Paul Jones – bass, electric piano, mandolin, recorders, synthesiser John Bonham – drums Producer - Jimmy Page

The one that's not really called 'IV' but is the fourth Led Zep album. The original LP had no text on the front or back sleeve. There's little about this album that hasn't already been said, but the tape-slowed doom blues of When The Levee Breaks is still its/Zeps best song.