Zep's debut is steeped in its own Blues tea, and there was/will never be another IV. There is no substitute for "When the Levee Breaks". I may want to hear it only every three years or so, but I gotta have it at the ready.
I and IV are just too played out, in the universe between my ears.
I never bought any Zep release after "Presence" (unless you count the "BBC Sessions"). The industry hyped "In Through the Out Door" like crack addicted apes. I was there for the FM radio/printed page hype, I heard it the album...and it stank.