Assuming, that Austria was reduced 1919 or 1945/1955 to a coherent country, losing the artificial appendage of northern Tyrol (becoming an independent BufferState between Italy & Germany) and the artificial appendage to the artificial appendage - Vorarlberg (to SwitzerLand).
What leaves a country easily divisible into 4 regions of roughly equal population:
...with perhaps 20 "Kreise" (=circuits), 5 per region:
Or alternatively these:
Below that level ~100 districts and some thousand municipalities would remain roughly unchanged.
All census- & election-numbers can be found here:
https://drive.google.com/drive/folders/19Do_q7eTST4nq02zKYE1Uy9GlMB7u-2M?usp=sharing