At the time Laachraoui was known as "Abou Idriss", a lawyer for two of the ex-hostages told French media.
The hostages were freed in April 2014.
Laachraoui and Ibrahim el-Bakraoui died when they attacked Brussels Zaventem airport a month ago. Their bombs and a third in the metro killed 32 people.
The confirmation about Laachraoui's role as an IS prison guard in Syria came from lawyer Marie-Laure Ingouf.
Laachraoui was a 24-year-old Belgian electrical engineer from the Brussels suburb of Schaerbeek. He had links to the jihadists who attacked Paris in 2015.
The French daily Le Parisien reports that Laachraoui kept watch over the four Frenchmen with Mehdi Nemmouche, the jihadist accused of killing four people at the Brussels Jewish Museum in May 2014. Nemmouche is now in Belgian custody.
The Frenchmen - Didier Francois, Pierre Torres, Edouard Elias and Nicolas Henin - spent 10 months in IS captivity. They had been captured in June 2013.
Laachraoui and Nemmouche disappeared at the end of January 2014, according to the ex-hostages.
One of the Brussels airport bombers, Najim Laachraoui, guarded four French journalists who were held hostage by so-called Islamic State (IS) in Syria in 2013-2014, the ex-hostages say.