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?