The body designation from GM for the Century/Regal body series was 4A from 1973 through 1981. In 1982 Buick introduced the front wheel drive Century and it retained the body designation 4A. The Regal became its own body series 4G from 1982 through 1987 and included the four door model and wagon for 82-84.
In 1979/80 you could buy the turbo motor in any Century model except the wagon, in the Riviera S or standard, but for some reason in the Regal and LeSabre it was limited to only the Sport Coupe models.
No body has ever come up with a number of turbo motors installed in standard versions of the Buicks during these years, but one of the turbo four door Centuries turns up every once and a while. I have only seen one.
The carb/tubo cars have yet to establish any collector status and odd balls like these four doors are so far under the radar that they aren't even carried in a price book.
If you like the car, buy it. And have fun driving something pretty rare that only other turbo Buick owners will understand. The only real value will come from how much you like it.
If you decide not to buy it, pass the information on. Some of us are always looking for an odd ball. That is one reason I hold on to my LeSabre Sport Coupe. You don't see too many.