I'm convinced that bands have some sort of a conspiracy about fourth albums.
Have you ever noticed that, when a band does their fourth album, it's often either very popular, very sucessful, very different, or very good? I first noticed this when I realized that A Night at the Opera is Queen's fourth album. As far as I know, A Night at the Opera is considered Queen's best album. It has their best song on it, anyway!
And then there's Led Zeppelin IV. It's a very famous Led Zeppelin album, and it has the world's number 1 rock song on it: Stairway to Heaven. I think that deserves it some special attention, doesn't it?
There's other albums, too: A Hard Day's Night, Eldorado, Leftoverture, and Fragile. A Hard Day's Night was a soundtrack of sorts to the Beatle's first movie. Leftoverture has Kansas's number one song, Carry On Wayward Son. Fragile was a breakthrough album for Yes (and, by extention, the entire prog genre). Eldorado, well, it's not too notable...but I love it, and remember, this is my blog, and if I like it, it goes on here. Also, if one considers Space Oddity to be David Bowie's first album, then that would make Ziggy Stardust his fourth. And we all know what that was famous for.
Ah! I've almost forgotten! Let me never forget that Tommy was the Who's fourth album. That album was almost revolutionary! (Heck, maybe it was!) It was the first album to be billed as a rock opera upon release! Let's never forget that!
So - is it a conspiracy? I think not. Sure, there's plenty of bands with fourth albums that were particularly good, but not enough to constitute a conspiracy. True, a lot of bands had some pretty good fourth albums, some of which are famous enough to say they're legendary. But there were plenty of bands that didn't fit into this pattern. (People like The Doors, Fleetwood Mac, the Rolling Stones, The Steve Miller Band, and Cream didn't have fourth albums that were that famous. And to diehard fans of these bands: I apologize if you don't agree with these descriptions. I've never even heard those guys' fourth albums! But if I haven't heard of them, they probably aren't too famous.) But then, I think that there might be enough evidence present to say that "fourth time's the charm" as far as music goes.