King of Prussia Frederick William IV, the eldest son and successor of Frederick William III of Prussia, reigned as King of Prussia from 7 June 1840 to his death on 2 January 1861. Also referred to as the "romanticist on the throne", he is best remembered for the many buildings he had constructed in Berlin and Potsdam as well as for the completion of the Gothic Cologne Cathedral.

In politics, he was a conservative, who initially pursued a moderate policy of easing press censorship and reconciling with the Catholic population of the kingdom. During the German revolutions of 1848–1849, he at first accommodated the revolutionaries but rejected the title of Emperor of the Germans offered by the Frankfurt Parliament in 1849, believing that Parliament did not have the right to make such an offer. He used military force to crush the revolutionaries throughout the German Confederation. From 1849 onward he converted Prussia into a constitutional monarchy and acquired the port of Wilhelmshaven in the Jade Treaty of 1853.

When revolution broke out in Prussia in March 1848, part of the larger series of Revolutions of 1848, the king initially moved to repress it with the army, but on 19 March he decided to recall the troops and place himself at the head of the movement. He committed himself to German unification, formed a liberal government, convened a national assembly, and ordered that a constitution be drawn up. Once his position was more secure again, however, he quickly had the army reoccupy Berlin and in December dissolved the assembly.

Following the revolutions of 1848, the increasingly gloomy king withdrew from the public eye, surrounding himself with advisers who preached absolute orthodoxy and conservatism in religious and political matters. A series of strokes from 14 July 1857 onward left the king partially paralyzed and largely mentally incapacitated.