Please answer a question about the following article about Cardinal (Catholicism):

In the year 1563 the influential Ecumenical Council of Trent, headed by Pope Pius IV, wrote about the importance of selecting good Cardinals. According to this historic council "nothing is more necessary to the Church of God than that the holy Roman pontiff apply that solicitude which by the duty of his office he owes the universal Church in a very special way by associating with himself as cardinals the most select persons only, and appoint to each church most eminently upright and competent shepherds; and this the more so, because our Lord Jesus Christ will require at his hands the blood of the sheep of Christ that perish through the evil government of shepherds who are negligent and forgetful of their office."

Who was the pope who headed the Ecumencial Council of Trent in that year?
Pope Pius IV