For progressive rock, if I had to limit it to just four bands, it’d be King Crimson, Yes, Genesis, and ELP. I’ve seen Pink Floyd and Jethro Tull listed along with the four bands as a big six of prog rock. Rush is a bit too late. By the time they released their first couple albums, King Crimson had already broken up, Peter Gabriel had already left Genesis, and ELP’s best days were behind them.
I don’t consider Pink Floyd a true progressive band as most of their music is slow, written in 4/4 time, with no complex melodies or instrumentals. I feel like the members of Pink Floyd would have had a hard time trying to play the music of most other prog bands including King Crimson and Rush. I’m not saying Pink Floyd isn’t prog, but they aren’t the greatest prog rock band ever or the definitive prog band. If progressive rock means sounding like King Crimson, Yes, Genesis, ELP, etc. then Jethro Tull, Rush, and even Kansas are more progressive than Pink Floyd.
Yeah Stone Temple Pilots was big in the 90s but I usually don’t seem them listed as one of the big four. It could be because they aren’t from Seattle, but then again, Anthrax isn’t from California yet they’re one of the big four of thrash.