River through four European capitals

DANUBE

This major waterway flows through Vienna and Budapest before emptying into the Black Sea, passing more countries than any other river in Europe.

This clue asks for a river that flows through four European capital cities. The Danube fits perfectly—it passes through Vienna (Austria), Bratislava (Slovakia), Budapest (Hungary), and Belgrade (Serbia), making it the only major river to do so. No wordplay here, just a straightforward geography fact.