You're right he has only had four masterpieces (the ones you listed as simply "good"): Alien, Blade Runner, Gladiator and Black Hawk Down. Then again, you phrased it as only. I could say:
Ridley Scott has FOUR masterpieces on his resume when most directors have none.
Then he has a long list of good, if not GREAT movies as well:
-Thelma and Louise
-Kingdom of Heaven (Director's Cut)
-Matchstick Men
-American Gangster
-The Duelists
-White Squall
Yes he does have some clunkers on his resume (I'm looking at you Hannibal, Robin Hood, A Good Year and Black Rain. But, so what?? He is not as consistent as Scorsese or even Spielberg, but he's made more great films than most directors including fanboy favorites around here like James Cameron and Peter Jackson (who I think both of whom are great, but are not above making bad movies).
Most filmmakers can't claim they've made so many quality films, especially ones that are considered classics, have completely reinvented genres in all four masterpieces cases, and have stood the test of time. This thread has haters gonna' hate written all over it, in my opinion.