At this moment, it is customary to pour another cup of wine that we do not drink during the Seder - a cup for the prophet Eliyahu, and briefly open the front door for him to come - his arrival is a sign of the beginning of the era of the Messiah, in which all oppression will end.

After that, the 4th cup of wine is filled.
Then it’s time to recite Hallel, after which we once again recite the blessing over the wine, this time for the last time at the Seder: “Blessed are You, Adonai our God, Ruler of the world, Creator of the fruit of the vine.”
Then drink the 4th cup and recite the final blessing over the wine (Maein Shalosh). 

Psalm 117
Praise Yahweh, all you nations; extol him, all you peoples. For his loyal love is mighty on our behalf,and the faithfulness of Yahweh is forever. Praise Yah! 

Psalm 118
Give thanks to the LORD, for he is good;
his love endures forever.
Let Israel say: “His love endures forever.”
Let the house of Aaron say: “His love endures forever.”
Let those who fear the LORD say: “His love endures forever.”