Elijah's Cup

Hi,

"One reason why we pour a fifth cup and call it 'Elijah's Cup' is that there is a debate in the gemara as to whether we use a fifth cup, and the halachah is not determined. Therefore, we pour a fifth cup and we call it 'Elijah's cup,' and when Elijah comes the doubt will be clarified."

(Taamei haMinhagim 551, citing the Vilna Gaon)

Have a great Moed,
Mordechai