Austrian postal codes are 4 digit. First digit is the region, 1 is Vienna, 2 and 3 lower Austria, 4 upper Austria, 5 Salzburg,...
Seond and third digit specify the region more closely, for example the district in Vienna. Last digit is the exact post office. 0 is the region's main or central post office.
So for example the 1st district, main post office is 1010. 1017 would specify a different post office. believed to speed up things, but I doubt it. 23rd district for example is 1230. Rural areas usually don't have the post office thing, they're mostly like 4720. These codes were introduced in 1965, prior to this only the bigger cities had postal codes (I only know about Vienna, they used a 2 digit system that didn't correspond to the districts), all smaller cities were just city + state.
So the correct way to address an international letter is like this:
Joe Average
Hauptstrasse 17/3 (=house number + apartment)
A 4710 Wildbach
Austria
(or Autriche, french is very common for postal affairs)
(Hauptstrasse means High Street, a very common name, postal code and city are freely invented)
