} |- | |} King Frederick William IV of Prussia (October 15, 1795 - January 2, 1861), the eldest son and successor of Frederick William III of Prussia, reigned as King of Prussia from 1840 to 1861.

Frederick William was educated by private tutors, many of whom were experienced civil servants. He also gained military experience by serving in the army during the War of Liberation against Napoleon I of France in 1814, though he was an indifferent soldier. He was a draftsman interested in both architecture and landscape gardening and was a patron of several great German artists, including architect Karl Friedrich Schinkel. He married Elisabeth Ludovika of Bavaria in 1823, but the couple had no children.

Frederick William was a staunch Romanticist, and his devotion to this movement, which in the German States featured a nostalgia for the Middle Ages, was largely responsible for him developing into a conservative at an early age. In 1815, when he was only 20, the crown prince exerted his influence to structure the proposed constitution of 1815, which was never actually enacted, in such a way that the landed aristocracy would hold the majority of the power. He was firmly against both liberalisation and unification of Germany, preferring to allow Austria to remain the principal power in the German states.

Upon his accession, he toned down the reactionary policies enacted by his father, easing press censorship and promising to enact a constitution at some point, but he refused to enact a popular legislative assembly, preferring to work with the aristocracy through "united committees" of the provincial estates. Despite being a devout Lutheran, his Romantic leanings led him to settle the Cologne church conflict by releasing the imprisoned Archbishop of Cologne, and he patronized further construction of Cologne Cathedral. In 1844, he attended the celebrations marking the completion of the cathedral, becoming the first king of Prussia to enter a Roman Catholic building. When he finally called a national assembly in 1847, it was not a representative body, but rather a United Diet comprising all the provincial estates, which had the right to grant taxes and loans but no right to meet at regular intervals.

When revolution broke out in Prussia in March 1848, part of the larger Revolutions of 1848, the king initially moved to repress it with the army, but later decided to recall the troops and place himself at the head of the movement on March 19. He committed himself to German unification, formed a liberal government, convened a national assembly, and ordered that a Constitution of the Kingdom of Prussia be drawn up. Once his position was more secure again, however, he quickly had the army reoccupy Berlin and dissolved the assembly in December. He did, however, remain dedicated to unification for a time, leading the Frankfurt Parliament to offer him the crown of Germany on April 3, 1849, which he refused, purportedly saying that he would not accept "a crown made of mud and clay". He did attempt to establish the Erfurt Union, a union of German states excluding Austria, soon after, but abandoned the idea by the Punctation of Olmütz on November 29, 1850, in the face of Austrian resistance.

Rather than returning to bureaucratic rule after dismissing the national assembly, Frederick William promulgated a new constitution that created a parliament with two chambers, an aristocratic upper house and an elected lower house. The lower house was elected by all taxpayers, but in a three-tiered system based on the amount of taxes paid so that true universal suffrage was denied. The constitution also reserved for the king the power of appointing all ministers, reestablished the conservative district assemblies and provincial diets, and guaranteed that the bureaucracy and the military remained firmly in the hands of the king. This was a more liberal system than had existed in Prussia before 1848, but was still a conservative system of government in which the monarch, the aristocracy, and the military retained most of the power. This constitution remained in effect until the dissolution of the Prussian kingdom in 1918.

A stroke in 1857 left the king partially paralyzed and largely mentally incapacitated, and his brother William served as regent from 1858 until the king's death in 1861, at which point he acceded the throne himself as William I.

*Frederick William IV and the Prussian Monarchy 1840-1862, by David E. Barclay, (Oxford, 1995).