I heard from Rabbi Avrohom Garden, that the whole thing of this possuk is
to protect us from all manner of dangers, if something is under your bed,
then it follows that the snake will have to come from under the bed and
over the side from one of the four directions, at that point one of the
malochim can take effect and protect us.
Ben Granat
Yeshivat Shaarei Yerushalayim