Big Si says:
So either first 3 or first 6, 4 seems a bit arbitrary.
I think it's because over the years, everyone seemed to talk about the first 4 so I assumed (possibly wrongly) whatever followed wasn't up to the same standard, whereas most Zeppelin fans will recommend anything up to 'Physical Graffiti'. I do admit few would name 'Houses of the Holy'.
If you asked someone to recommend a Sabbath album I doubt they'd name anything after 'Vol 4'. I think this is the one Ben recommended in the pub but I was quite drunk and spilled a half pint of beer on his crotch shortly after the conversation.
Thankfully Spotify seems to have most of the early stuff so I can fill in some gaps.