Final answer:
The Danube River flows through Vienna, Bratislava, Budapest, and Belgrade. It is one of the major rivers in Europe and has influenced the transportation and trade history of the continent.
Step-by-step explanation:
The river that flows through the four capitals, Vienna in Austria, Bratislava in Slovakia, Budapest in Hungary, and Belgrade in Serbia, is the Danube River.
The Danube River is one of the major rivers in Europe, flowing east through several European countries. It starts in Germany and passes through or forms part of the borders of ten countries before emptying into the Black Sea.
The Danube River has played a significant role in the transportation and trade history of Europe, and its beautiful banks and cities attract tourists from around the world.