King of Prussia from 1840 to 1861. Perhaps the most intelligent and artistically talented Prussian monarch, he proved to be an erratic and unreliable leader during the German Revolution of 1848.
Frederick William’s ascension to the throne on June 7, 1840, was greeted with the expectation that he might help to realize the liberal-national aspirations of his distinguished friends. He soon alleviated press censorship and affirmed religious freedom for the independent Protestant sects and Rhineland Catholics. Yet personally he was devoted more to the ideals of the Holy Roman Empire and divine right of kings than to liberal constitutionalism, and he disillusioned liberals by delaying the promulgation of a constitution, which had been promised by his father. He finally yielded to pressure in February 1847, but rather than a popularly elected body he called only a united Landtag (diet)—a group of delegates from the traditional provincial diets.
With the outbreak of violence in March 1848 in Berlin, the King immediately lost his nerve and capitulated to the rebels, even to the point of riding through the streets of Berlin under the revolutionary German flag. But as soon as his armies had gained control again, he betrayed his promises, dissolved the popular assembly established by the revolution, and proclaimed a new reactionary constitution in December 1848. When the revolutionary all-German Parliament in Frankfurt offered him the imperial crown, he rejected it for ideological and political reasons as “unworthy.” A subsequent attempt by his adviser J. von Radowitz to create a union of German princes under Prussian leadership failed when combined pressure by Austria and Russia forced Frederick William to capitulate at Olmütz (1850).
During the remaining years of his reign the King withdrew increasingly to his artistic pursuits and left politics more and more in the hands of the ministers of the reaction. After he suffered a stroke in October 1857 and consequent mental collapse, his brother William ruled as regent until Frederick William’s death in Potsdam on 2 January 1861.

Link to Wikipedia biography