John IV was the eldest son of John III of Gaeta

John III of Gaeta

John III was the consul and duke of Gaeta from some time between October 984 and January 986 until his death.He was the eldest son of Marinus II, who succeeded his brother Gregory in 978 and immediately appointed John as co-duke in order to assure his inheritance, as the precedent of fraternal...

 who was appointed co-duke in 991 while still young (he was a minor even in 994). John succeeded his father in 1008 or 1009 and ruled for a brief four years.

John married Sichelgaita, daughter of John IV of Naples

John IV of Naples

John IV was the mostly absentee duke of Naples from 997 to after 1002. He was the son and successor of Sergius III. John IV originally recognised the suzerainty of the Byzantine Emperor....

 and sister of Sergius IV

Sergius IV of Naples

Sergius IV was Duke of Naples from 1002 to 1036. He was one of the prime catalysts in the growth of Norman power in the Mezzogiorno in the first half of the eleventh century...

. He left an infant, perhaps posthumous, son under the regency

Regent

A regent, from the Latin regens "one who reigns", is a person selected to act as head of state because the ruler is a minor, not present, or debilitated. Currently there are only two ruling Regencies in the world, sovereign Liechtenstein and the Malaysian constitutive state of Terengganu...

 of his mother, Emilia

Emilia of Gaeta

Emilia was the duchess of Gaeta first as consort of John III and then as the regent for her grandson John V until at least 1029....

. His brother Leo

Leo II of Gaeta

Leo II was the regent and tutor of John V of Gaeta in opposition to his own mother, Emilia, from 1014 to January 1025. He undersigned documents as consul et dux. He was the son of John III of Gaeta and thus uncle of John V....

 fought for the regency and his cousin Leo

Leo I of Gaeta

Leo I, called the Usurper, was the usurping consul and duke of Gaeta on two quite separate occasions. He was the son of one Docibilis, a relative of the Docibilian dynasty, probably a son of Duke Gregory....

 tried to usurp the throne from the young John V

John V of Gaeta

John V was the consul and duke of Gaeta from 1012 to 1032. He was the son of John IV and Sichelgaita, sister of Sergius IV of Naples. He was either very young when he succeeded his father or perhaps he was even born posthumously.His regency was disputed by Leo, his father's brother, and the...

.

Sources

• Caravale, Mario (ed). Dizionario Biografico degli Italiani LV Ginammi – Giovanni da Crema. Rome
  Rome
  Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...
  
  , 2000.