Missed this thread .....but I have only heard big 6 mentioned and not just 4,,,,at least that is how its always presented on Prog Rock Archives and by progheads.
The UCR link only mentions 4....which is a bit odd since it has usually been these 6.
I think Jerry Lucky coined the term in the early 80's in his book 'The Progressive Rock Files'.
The Big 6 of progressive rock are Yes, Genesis, King Crimson, Emerson, Lake & Palmer (ELP), Pink Floyd, and Jethro Tull. Some at PA will substitute Rush or Gentle Giant.
The 50 top prog lp list from UCR is pretty solid though there are a few lps I would not put on the list and a few I would......but that's for another day.