Well, a sedan is going to be heavier than a two door convertible for starters.
Let's see, assuming the car is made of steel of adequate thickness amd carbon content -
Two rear doors, seats, seatbelt hardpoints, chassis supports for the above - 700lbs.
Hardtop roof for sedan plus mandatory side curtain airbags - 350lbs
Stronger springs to support greater weight of car - 40lbs.
Longer wiring harness - 20-40lbs, depending on system.
An interesting comparison - my 1987 Series III tips the scales at 4400lbs. The new XJ8 Vanden Plas, despite being made largely out of aluminum, lots of weight saving measures, and only a few additional "amenities" weighs 3900lbs. Despite the fact that the engine weighs less than half as much as mine, the transmission is also lighter, etc.,etc. They spent a lot of time and money to make the thing lighter, but the government mandated idiocy still makes it only about 500lbs lighter, which is pretty sad considering that if you stripped it of the reinforcements and airbags you'd be looking at a 2900lb sedan (someone's already done this with a flood car).
The first generation Miata is 2100lbs. They managed to hold the weight down on successive generations by spending more money on more exotic materials... and the price of the Miata sure shows it.