Although carrying over the same platform, mechanicals, and some body panels seen on the "Third Generation" Riviera, Buick replaced its distinctive 'boat tail' roofline with a more conventional-looking "Colonnade" treatment. This turned the car from a hardtop coupe into a pillared coupe, as it featured wide B pillars and fixed quarter opera windows. A landau half-vinyl roof option was available. The car did retain its forward-jutting grille, albeit in slightly modified form. Thus modified, the car looked far less distinctive than its predecessors, and even its platform mates, the Oldsmobile Toronado and Cadillac Eldorado. The tamer-looking Riviera was no lighter, and its standard 455 V8 lost more power, dropping to 230 hp (172 kW) and 245 hp (183 kW) for standard and Stage One models, respectively. Max Trac was dropped from the option list after 1974 due lack of buyer interest. The revised styling did nothing for sales, which plummeted to 20,129 in 1974.
For 1975, the Riviera received an updated front fascia, which lost its forward-jutting theme through the redesign of the fiberglass front end cap. Quad rectangular headlights were mounted horizontally. The new vertical-bar grille echoed the "stand-up" theme that many GM cars of the day incorporated. Parking lights wrapped around the fender sides. The Stage One performance package was dropped for 1975, though the Gran Sport handling package would continue to be offered. The standard engine's output dipped to 205 hp (153 kW). Sales for 1975 were 17,306.
Minor changes greeted 1976 models, the most notable of which was a new crosshatch grille insert. The Gran Sport handling package was replaced by an 'S/R' package that had similar sporting pretensions. Sales rallied slightly to 20,082 for 1976.
Source: Wikipedia, 2011