Interesting. My 'big 4' of prog rock would be King Crimson, Yes, Genesis AND Pink Floyd - for sure not Rush, even though I like them more than Genesis and also more than most Floyd (except for Wish You Were Here). I mean, Rush are great but they arrived late to the scene and were outliers in several ways. I don't think they can be put in the same league.
Usually I had heard about the 'big six' of prog (King Crimson, Yes, Genesis, Pink Floyd, ELP and Jethro Tull). I've seen that mentioned in multiple occasions. But then there's the perennial question of whether Floyd (and even Tull) are genuinely prog or not.