Police discovered two cylinders of gas in the hall of a building in the city's 16th arrondissement and two others on the sidewalk. They also found a mobile phone linked to the cylinders, which appeared to be "a device to ignite" the explosive, the sources said.
A local resident alerted police after finding two of the gas cylinders around 4:30 am (0230 GMT) on Saturday, a source close to the probe said.
According to another source, four of the suspects were being held in custody Monday night.
The latest attack was on Sunday in Marseille when a knifeman stabbed two women to death at the main train station. He was shot and killed by soldiers.
The stabbings bring to 241 the number of people killed in terror attacks in France.