4 wheel disc brakes? was that even an option. https://www.hemmings.com/classifieds/dealer/buick/riviera/2224739.html
Yes, an option for those '77 and '78 Rivieras. The same 8.5" rear used for the cadillac Fleetwood of the same years.
Yes rear disc brakes were also on the '76 - '79 Cadillac Seville, as well. The Seville used a stretched X body chassis ('75 - '79 Nova, Ventura/Phoenix, Omega, and Apollo/Skylark). Car in Hemmings ad has a 403 oldsmobile engine. Possibly a California car. Both '77-'78's that saw in the El Paso U Pull It's awhile ago were 350 Buick cars.
Also Trans Am's had 4 wheel disc & most came with a 403 Olds '76-'79. My sister bought a '78 T/A new. At the time she specified she wanted a 400 Pontiac engine & Bird delete. It took 4 times to get it right. The 4th. one didn't have Bird delete, since it had the RIGHT engine she had them take it off & repaint the hood. She still has the T/A today & it has ONLY accumulated 30K miles. So there's 3 Metallic Blue '78 T/A's out there she ordered with 403 Olds engines. Tom T.
I had a ‘79 T/A, W72/WS6. It was the 400 Pontiac, Super T 10, and four wheel discs, I bought it from the original owner who’d special ordered it as a hood bird delete car. Patrick