This refers to Vienna which is divided into 23 districts, each with a 4 digit postcode beginning with "1" (for Vienna), i.e. 1010 (first district - city centre), 1020 (second) etc. throught to 1230 (23rd). Last digit can also be more clearly defined as an area within the district although this generally tends to be left as "0".

There, only a year late, but hopefully useful for any future query!

just like in Budapest. :-)