Here are some other Five-Album Runs i consider to be pretty good, if not great:
The Flower Kings: Back In The World Of Adventures -> Space Revolver - their first five albums are all really amazing in their own ways, lots of classic TFK songs in those albums!
Journey: Infinity -> Frontiers - their greatest hits all come from these five albums, and it's pretty amazing considering they were all released in 6 years! These are about the only 5 Journey albums I ever listen to anyway.
King Crimson: Larks' Tongue In Aspic -> Beat - the Wetton-era, for me, is one of the best 3-album runs in prog history, and I've never really gotten into Lizard or Islands, but the first two 80's KC albums are pretty stellar, so I'm listing these five here.
Led Zeppelin: Led Zeppelin -> Houses Of The Holy (and maybe Physical Graffiti too) - LZ's first five albums are staples of classic rock, and while there are a couple of "just okay" songs here or there, as a whole run of five albums, this is pretty great in my opinion.
Marillion: Script For A Jester's Tear -> Season's End - while I haven't personally listened to Fish-era Marillion on quite some time, I will say that I recall all four of those albums being really good, as was the first album with H. These five albums are the height of 80's Neo-Prog, and I think they should definitely be mentioned as one of the better five-album runs.
One Honorary "Almost Made It" Mention - Neal Morse: If it weren't for the lackluster Lifeline, any run of Neal's albums between Testimony and The Similitude Of A Dream could quality, but Lifeline finds itself smack dab in the middle of 9 albums. Perhaps with Neal's next album, the run of Testimony 2 -> __________ could claim a spot as one of the great five-album runs?
Now, if we're talking strictly ANY album Neal has been a part of, his 2000-2003 run of SMPT:e - V - Bridge Across Forever - Snow - Testimony would be great, but it cuts out It's Not Too Late. There's also the 2009-2014 run of The Whirlwind - Testimony 2 - Flying Colors - Momentum - Kaleidoscope - Second Nature is phenomenal - 2 Transatlantic albums, 2 Flying Colors albums, and 2 of his best solo albums.
-Marc.