However, all four - Salim Jamil Ayyash, Mustafa Amine Badreddine, Hussein Hassan Oneissi and Assad Hassan Sabra - remain at large and are bring tried in absentia.
The UN-backed trial in The Hague opened against a backdrop of continuing sectarian violence in Lebanon.
A car bomb exploded early yesterday close to the country's border with Syria, killing at least three people and wounding more than 20.
Mr Hariri's son Saad - like his late father, also a former prime minister - was in The Hague to attend proceedings along with family members of other victims of the blast, which happened on February 14, 2005.
The suspects have not been arrested and Shiite group Hezbollah denies involvement in the murder.
The group's leader, Sheik Hassan Nasrallah, has denounced the court as a conspiracy by his arch enemies - America and Israel.
Presiding Judge David Re says prosecutors will call hundreds of witnesses in a trial likely to take months.
His courtroom was dominated by a large scale model of the Beirut street where the blast happened.