This place in France has advertised a 1981 GT4... http://www.carugati.ch/index.php?page=&lang=fr You'll have to view the inventory to find the car - the above link is not direct to the car. Incidentally, the picture is probably the only time you'll ever see a GT4 parked that close to a Pagani Zonda!
Looks like they are posting the mo/yr of manufacture....so perhaps a 1980 model year, produced in early 1981? All the books that I have read are not month specific relative to when production stopped, but I have never seen anything referenced as a 1981 Model Year. Maybe a late serial number car. Another note: 45000 Swiss Franc is about $35K US.
