The raids came a day after it emerged a Frenchman was being held by police investigating the murder of three people at the Brussels Jewish Museum.
Mehdi Nemmouche, 29, was arrested at a station in Marseille on Friday.
Prosecutors say he has claimed responsibility for the attack and spent more than a year in Syria.
Interior Minister Bernard Cazaneuve told Europe 1 radio on Monday: "There are people who recruit jihadists. There are as I'm speaking arrests being made."
"We are acting everywhere. There will be no respite in the fight against terrorists."
There is no suggestion of a link between the four arrests on Monday and the detention of Mr Nemmouche during a random check on a coach arriving from Amsterdam in southern France on Friday.
But Mr Nemmouche is said to have had links with radical Islamists and served five years in jail in France for robbery before being released in December 2012.
When he was arrested, he had with him a Kalashnikov rifle and a handgun believed to have been used in the attack, the Paris prosecutor said.
He was also said to have had a white sheet emblazoned with the name of the Islamic State of Iraq and the Levant, a jihadist group fighting in Syria, and a camera with a 40-second video showing the two guns and a voice recording, claiming responsibility for the killings.
Speaking on French radio station RTL on Monday, the head of French Jewish association CRIF, Roger Cukierman, called for more resources to be given to the foreign intelligence service, the DGSE, to track militants returning to France from Syria.
He feared "they would become 700 time bombs when they return", referring to the estimated number of French-born jihadists in Syria.
Belgium has requested Mr Nemmouche's extradition from France and police have to decide whether to extend his detention until Thursday.
Three people died when a gunman opened fire at the museum in the busy Sablon area of the Belgian capital on 24 May. They were an Israeli couple in their 50s, and a French female volunteer.
A Belgian man, believed to be an employee of the museum, was critically injured.
Four people have been arrested in the Paris region and southern France, officials say, on suspicion of recruiting militants to fight in Syria.