The Gaon of Vilna understood the fourth cup to signify the olam haba – the world to come – an end time whereupon the righteous are finally and fully delivered by Hashem. According to one mystical approach, the ushering of this epoch requires the transformative union of livyatan and b’hemot, ur-creatures of the sea and land respectively. As a cup is typically constructed of two circles, one above and one below, these animals are each represented as an ouroboros – a serpent and a bull consuming their tails, together symbolizing that all of creation and our existence will come to full circle and utter completion – shalem.