Jalal Talabani, Iraq's president, and al-Maliki were flanked by Massoud Barzani, the leader of the northern autonomous Kurdish region and Adel Abdul-Mahdi, the Shia vice-president as the announcement was made.
The four men signed a three-page agreement that they said would ensure them a majority in the 275-member parliament and allow the passing of critical US-demanded legislation.
Hoda Abdel-Hamid, Al Jazeera's Iraq correspondent, said: "The new alliance gives them the ability to continue passing legislation. However without Sunni representation ... it could increase sectarian violence."
Talabani, a Kurd, said that al-Hashemi had refused the invitation to join in the new political grouping but that "the door is still open to them and they are welcome at any time".
Al-Maliki also called on the Sunni Accordance Front, which includes al-Hashemi's party, to return to the government after five of the bloc's cabinet ministers quit the government.
The four-party agreement comes four weeks before David Petraeus, the most senior US commander in Iraq, and Ryan Crocker, US ambassador to Iraq, are due to deliver a progress report on Iraq to congress.
In a separate development, a car bomb has struck a parking garage in a central commercial district of Baghdad, killing at least nine people and wounding 17.
The bomb went off during Thursday's morning rush hour near the busy Rusafi Square.
Smoke poured out of the seven-storey concrete building and food and merchandise stalls below were left charred.
Source: Al Jazeera and agencies