A woman is pregnant with quadruplets; is it permissible for her to abort two?

There is a woman who cannot have children. The doctors examined her and discovered that her eggs were dead. They did an operation to transplant eggs and she became pregnant with four embryos. When the doctors discovered this and told her husband, he said that he did not want four children, because his wife might get too tired. He told them to abort two of them and leave two. The doctors said, “We will not do that unless we have a fatwa from Shaykh Muhammad ibn Saalih al-‘Uthaymeen.” The pregnancy has already lasted three months, so is it permissible for the doctors to do this operation, since they have agreed to wait for your opinion?