The victims were trampled at a beach in the capital Conakry as thousands surged to the narrow exits at the end of a performance by rap group "Instinct Killers" marking the close of the Muslim holy month of Ramadan on Tuesday.
A death toll of 33 was raised by hospital authorities after another concert-goer died of head injuries overnight.
The judicial source told AFP an investigation launched after the accident, which injured around 60 people, was progressing and that "there have been four arrests".
They were interviewed by police today, public prosecutor Aboubacar Sylla told AFP.
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"All authors and makers of the crisis will be... Tried so that the people of Guinea know what happened," Sylla said.
Guinea has been observing a week of national mourning since the tragedy, and the government has banned seafront public events.
Conakry, which sits on the tip of a peninsula jutting into the Atlantic Ocean, boasts miles of coastline as well as the nearby Iles de Los, islands which are popular with expat beach-goers.