Each of the two graves contained two bodies, Xinhua news agency quoted the General Authority for Research and Identification of Missing Persons as saying in a statement on Wednesday.
Search is underway for more mass graves in the city, the statement said.
The forces of the UN-backed Libyan government have been accusing the rival eastern-based army of mass killings in Tarhuna, where dozens of unidentified bodies have been recovered so far.
The two sides had been engaged in a deadly armed conflict for more than a year in and around Tripoli, which ended recently with the UN-backed government taking over all of western Libya after the withdrawal of the eastern-based army.
Tarhuna was once the eastern-based army's main military operation centre.
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