They are very close. My 95 RMS weighed around 4300lbs with steel wheels, my 94 Caprice 9C1 weighed around 4200lbs with steel wheels. They both had the same driver, and a similar amount of gas in the trunk. I'm sure some of the Roadmasters options added to that weight. Power seats, cruise control, class III hitch... etc.
All depends on the options. The cars are fundamentally the same and, with the same options, there's no real difference in weight with both around 4220. Even a loaded up FWB only runs around 4400lbs(sources: Consumer Guide, Wiki, GM)