All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
If the existence of the four embryos leads to critical reprusccusions/or threat on mother's health, life or a possibility of aborting all of them, then aborting one/two/three is permissible mitigate the above mentioned risks so long as the ages of the embryos don't exceed four months. And Allah Knows Best.