The source told Reuters that one of those held has been charged with a terrorism-related offense and placed in custody in the southwestern Gironde region.
On Sept. 25, a stabbing in which two journalists were wounded took place in Paris's rue Nicolas-Appert, where Islamist militants killed 12 people in the offices of satirical magazine Charlie Hebdo five years ago.
The main suspect in the attack was an 18-year-old man from Pakistan. Police arrested him soon after the stabbing.
French daily Le Parisien reported on Friday that the four detained people of Pakistani origin had allegedly encouraged the attacker to act.Several attacks in recent months, including the beheading of a teacher who had shown his class caricatures of the Prophet Mohammad, have prompted the French government to take action against Islamist "separatism."