Eliyahu HaNavi is the song sung at the opening of the door for Elijah, upon pouring the fourth cup.

Eliyahu HaNavi

"Eliyahu HaNavi", meaning Elijah the Prophet in English, is a famous Hebrew song. It's an entreaty to prophet Elijah, who is an invited guest at the Passover meal, to return soon with the Messiah. This traditional Jewish song also forms a part of the traditional Saturday night Havdalah service. This song is to meant be sung when the Cup of Elijah is poured out and the door is opened in anticipation of his return. Elijah the Prophet was one of the greatest Jewish prophets. He served as the inspiration for the birth of the Carmelite Order. Elijah the Prophet was one of those rare men to be given a place in heaven even before death embraced him. That is why he is known as the "Angel of the Covenant." According to the Jewish legends, when a new Jew soul is born into the world, Eliyahu HaNavi presides over his circumcision. This Passover song is also a famous lullaby. Here is the Hebrew and the English translation of the song.

Eliyahu HaNavi - Elijah, The Prophet
Eliyahu HaNavi (Hebrew)
Eliyahu haNavi
Eliyahu haTishbi,
Eliyahu haGil'adi -
Bim'hera v'yameinu yavoh eleinu,
im mashiach ben David.

Elijah the Prophet (English)
Elijah the Prophet,
Elijah the Tishbite,
Elijah, Elijah,
Elijah the Gileadite
Speedily and in our days,
Come to us,
With the messiah, son of David,
With the messiah, son of David.

Prophet Elijah represents the humble wayfarer and during the Passover, doors are left open to welcome him. After the door is opened, the family member, usually the eldest member or Seder leader prays, like a request to God, asking for his protection for the Jewish people from evil. According to an age-old tradition, Eliyahu HaNavi appears surreptitiously at every Pesach Seder. Although there is no source to affirm that, most sources point towards an eschatological appearance at the End of Days, i.e., Eliyahu's arrival to announce the coming of Mashiach. The most common custom is to take a large decorative cup, famously called as Kos shel Eliyahu or Eliyahu's Cup, fill it with wine, and leave it on the Seder table.