The four went on trial in absentia before the Special Tribunal for Lebanon last month for the massive 2005 car bombing that killed billionaire Hariri and 22 others, including a suicide bomber.
Although the attack was initially blamed on pro-Syrian Lebanese generals, the court in 2011 issued arrest warrants against Mustafa Badreddine, 52, Salim Ayyash, 50, Hussein Oneissi, 39, and Assad Sabra, 37, all members of the Syrian- and Iranian-backed Shiite movement Hezbollah.
Merhi is charged with coordinating the preparation and delivery of a video claiming false responsibility for the attack that was sent to pan-Arab broadcaster Al-Jazeera with a view to protecting the real killers.
His case will now be added to that of the four others, STL judge David Re said today.
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"The interests of justice must favour joining the two cases," Re said, according to a statement from the court based just outside The Hague.
"The trial chamber has therefore decided that the two cases should be joined and tried on the same indictment," the judge said.