Album sometimes referred to as "Four Symbols" or "Zoso" because its official title is a series of four symbols representing the four members of the band, one of which appears to be the word "Zoso." Zeppelin had to supply the symbols to Billboard magazine so that the album could be listed on the charts. But with the previous albums Led Zeppelin I, Led Zeppelin II, and Led Zeppelin III, most people didn't bother with the symbols when speaking of the fourth album.