1998 Buick Riviera SuperCharge
My Buicks been sitting for about 4 years.
When I parked it, it started fine. Mechanic
guessed fuel pump. What else should I
That's a long time for one of those to sit. Yes on the fuel pump, fuel filter and relay. Before you get to far along I would, remove all the spark plugs and squirt just a small amount of oil in the cylinders and then unplug the ignition module and spin it over with the starter for a few seconds, then if the plugs are good put them back in or replace them, reconnect the ignition module, service the engine oil and filter. After you get it running I would then drain the cooling system, replace the thermostat and refill it with new coolant. then service the transmission fluid and filter. all that before you even try to drive it.
In addition to all the above great suggestions, you should consider removing what gas there is from the gas tank and clean the tank before starting the vehicle.