The Fifth Cup
The first four promises of liberation from Egyptian bondage and that of "I will take you to Me for a people" through giving them the Torah were completely fulfilled for the generation of our ancestors to whom these promises were given. But the fifth one was not – an entire generation died on the way to their promised land because of their sins!
The answer to this puzzle is that the first four promises were given unconditionally in contrast to the fifth one which was preceded by the condition "You shall know that I am the L-rd your G-d", a condition they failed to fulfill by accepting the slander of the spies about the land.
Knowing G-d was and remains our guarantee of Israel forever.