Frederick was deemed a man of responsibility and industry — often regarded as the most intelligent of Denmark-Norway's absolute monarchs.
He seems to have mastered the art of remaining independent of his ministers.
Lacking all interest in academic knowledge, he was nevertheless a patron of culture, especially in art and architecture.
His main weaknesses were probably pleasure-seeking and womanising, which sometimes distracted him.
He was the second to last Danish king to make a morganatic marriage (the last was Frederick VII with Louise Rasmussen aka "Countess Danner").

  
Without divorcing his first queen, Louise, in 1703 he married Elisabeth Helene von Vieregg (d.1704).

After the death of Elisabeth, he entered a romance with her lady-in-waiting Charlotte Helene von Schindel, though he later lost interest in her.
In 1711, Frederick then fell in love with 19-year-old Countess Anne Sophie Reventlow, daughter of the then Grand-Chancellor Conrad von Reventlow.
He carried her off from her home, Clausholm Castle near Randers, after her mother refused to let her daughter be a royal mistress.
Frederick had seen Anne Sophie at a masquerade ball at Koldinghus, where the royal family resided during the plague that devastated Copenhagen.
A secret marriage was held at Skanderborg on 26 June 1712.
At that time he accorded her the title "Duchess of Schleswig" (derived from one of his own subsidiary titles).
Three weeks after Queen Louise's death in Copenhagen on 4 April 1721, he legalized his relationship with Anna Sophie by a new marriage, this time declaring her queen consort (the only wife of a hereditary Danish king to bear that title who was not a princess by birth).
It was undoubtedly a relief to get out of a relationship they both saw as sin.
Of the nine children born to him of these three wives, only two of them survived to adulthood: the future Christian VI and Princess Charlotte-Amalia, both from the first marriage.
All the other children died in infancy.

The Reventlows took advantage of their kinship to the king.
Anna's sister, the salonist Countess Christine Sophie Holstein of Holsteinborg, was nicknamed Madame Chancellor because of her influence.
Within a year of making Anna queen, Frederick also recognized as dynastic the issue of the morganatic marriages of two of his kinsmen, Duke Philip Ernest of Schleswig-Holstein-Glucksburg (1673–1729) and Duke Christian Charles of Schleswig-Holstein-Plön-Norburg (1674–1706), to non-royal nobles.
The other Schleswig-Holstein dukes of the House of Oldenburg perceived their interests to be injured, and Frederick found himself embroiled in their complicated lawsuits and petitions to the Holy Roman Emperor.
Also offended by the countess's elevation were Frederick's younger unmarried siblings, Princess Sophia Hedwig (1677–1735) and Prince Charles (1680–1729), who withdrew from Copenhagen to their own rival court at the handsomely re-modelled Vemmetofte Cloister (later a haven for dowerless damsels of the nobility).
Frederick was deemed a man of responsibility and industry — often regarded as the most intelligent of Denmark-Norway's absolute monarchs. He seems to have mastered the art of remaining independent of his ministers. Lacking all interest in academic knowledge, he was nevertheless a patron of culture, especially in art and architecture. His main weaknesses were probably pleasure-seeking and womanising, which sometimes distracted him. He was the second to last Danish king to make a morganatic marriage (the last was Frederick VII with Louise Rasmussen aka "Countess Danner"). 
  
Without divorcing his first queen, Louise, in 1703 he married Elisabeth Helene von Vieregg (d.1704). 
After the death of Elisabeth, he entered a romance with her lady-in-waiting Charlotte Helene von Schindel, though he later lost interest in her. In 1711, Frederick then fell in love with 19-year-old Countess Anne Sophie Reventlow, daughter of the then Grand-Chancellor Conrad von Reventlow. He carried her off from her home, Clausholm Castle near Randers, after her mother refused to let her daughter be a royal mistress. Frederick had seen Anne Sophie at a masquerade ball at Koldinghus, where the royal family resided during the plague that devastated Copenhagen. A secret marriage was held at Skanderborg on 26 June 1712. At that time he accorded her the title "Duchess of Schleswig" (derived from one of his own subsidiary titles). Three weeks after Queen Louise's death in Copenhagen on 4 April 1721, he legalized his relationship with Anna Sophie by a new marriage, this time declaring her queen consort (the only wife of a hereditary Danish king to bear that title who was not a princess by birth). It was undoubtedly a relief to get out of a relationship they both saw as sin. Of the nine children born to him of these three wives, only two of them survived to adulthood: the future Christian VI and Princess Charlotte-Amalia, both from the first marriage. All the other children died in infancy.
The Reventlows took advantage of their kinship to the king. Anna's sister, the salonist Countess Christine Sophie Holstein of Holsteinborg, was nicknamed Madame Chancellor because of her influence. Within a year of making Anna queen, Frederick also recognized as dynastic the issue of the morganatic marriages of two of his kinsmen, Duke Philip Ernest of Schleswig-Holstein-Glucksburg (1673–1729) and Duke Christian Charles of Schleswig-Holstein-Plön-Norburg (1674–1706), to non-royal nobles. The other Schleswig-Holstein dukes of the House of Oldenburg perceived their interests to be injured, and Frederick found himself embroiled in their complicated lawsuits and petitions to the Holy Roman Emperor. Also offended by the countess's elevation were Frederick's younger unmarried siblings, Princess Sophia Hedwig (1677–1735) and Prince Charles (1680–1729), who withdrew from Copenhagen to their own rival court at the handsomely re-modelled Vemmetofte Cloister (later a haven for dowerless damsels of the nobility).
Frederick IV of Denmark