The verdict comes as Lebanese are still reeling from the aftermath of a huge explosion that killed 178 people this month and from an economic meltdown that has shattered their lives.
Hariri's assassination plunged Lebanon into what was then its worst crisis since the war, setting the stage for years of confrontation between rival political forces.
The Iran-backed Shi'ite Muslim group Hezbollah has denied any involvement in the Feb. 14, 2005 bombing.
'International Justice Defeats Intimidation' read a headline in Lebanon's an-Nahar daily with a caricature of the slain Hariri's face looking at a mushroom cloud over the devastated city, with a caption: "May you also (get justice)", referring to an investigation that could unveil the cause of the blast.
Hezbollah leader Sayyed Hassan Nasrallah said on Friday that if any members of the movement were convicted, the group would stand by their innocence.
But views of the case are mixed.
Beirut tour guide Nada Nammour, 54, speaking before the reading of the verdict began, said that the 2005 bombing was a crime that should be punished. "Lebanon needs to see law and justice ... We were born in war, we lived in war and will die in it, but our children deserve a future."
The verdict in The Hague may further polarise the already divided country and complicate an already tumultuous situation after the Aug 4 blast at Beirut port, where authorities say ammonium nitrate stored unsafely detonated, fuelling public outrage and leading to the government's resignation.
Harri's killing removed a powerful Sunni leader and allowed the further political expansion of Shi'ite power led by Hezbollah and its allies in Lebanon.
Justice 15 years on
The judgment had initially been expected earlier this month, but was delayed after the port explosion.
The investigation and trial in absentia of the four Hezbollah members has taken 15 years and cost roughly $1 billion. It could result in a guilty verdict and later sentencing of up to life imprisonment, or acquittal.
DNA evidence showed that the blast that killed Hariri was carried out by a male suicide bomber who was never identified.
Prosecutors used cell phone records to argue the men on trial, Salim Jamil Ayyash, Hassan Habib Merhi, Assad Hassan Sabra and Hussein Hassan Oneissi, carefully monitored Hariri's movements in the months leading up to the attack to time it and to put forward a fake claim of responsibility as a diversion.
Court-appointed lawyers said there is no physical evidence linking the four to the crime and they should be acquitted.
The reading of the verdict is set to last several hours.
Hariri's son Saad, who took his father's mantle and has served as premier three times, is expected to attend. He has said he was not seeking revenge, but that justice must prevail.
Some Lebanese say they are now more concerned with finding out the truth behind the Beirut port blast.
"I do want to know what the verdict is ... but what matters now is who did this (port blast) to us because this touched more people," said Francois, a volunteer helping victims in a ruined district.