The eldest son Henry IV was to represent his younger brothers John and Jošt II.
Jošt II, being the second son, had joined the clergy, so when Henry IV died in 1457, John came to rule the Rosenberg estates.
They won and on 15 November 1458 a peace treaty was signed, which obligated the City of Jihlava to pay tribute to King George.
On 31 August 1459, he pledged a large part of his possessions to his brother Jošt II, though this may have been a sham transaction to confirm the solvency of the Rosenbergs.
Pope Pius II had allowed the Hussite George of Poděbrady to be King of Bohemia, in the interest of European peace.
Sixteen of the most influential Catholic aristocrats in Bohemia, John and Jošt II were among them, gathered on 28 November 1465 at Zelená Hora Castle, which was owned by Zdeněk of Sternberg.
They founded the Zelená Hora Alliance, led by Zdeněk and wrote a pamphlet accusing George of violating the law of the land.
When John saw the intransigence and unwillingness to compromise on the part of the Catholic Alliance against the king, he joined the Royalists again, in 1466.
In early October, he signed a preliminary peace treaty with Zdeněk of Sternberg and Henry IV of Neuhaus.
After John's army was weakened and his finances were running low, he agreed in the summer of 1468 to hold peace negotiations with the Alliance.
John Popel of Lobkowicz and his son Děpolt were captured and were held prisoner in Český Krumlov Castle.
After George of Poděbrady died in 1471, John received from Matthias Corvinus the Lordships of Bechyně and Kouřim and territories along the Vltava.