Although 4 wasn't commercially successful, it massively helped Beyonce move her public image and fame to a place where she COULD tour with no album then surprise release an album half-way throught the tour and have people eating out of her hands the entire time.
Born This Way, I believe. 4 is perhaps more respected, but Born This Way is on the books for its lead single and its promo cycle. The last big "event" album before Bey's most recent.