The four were considered the military arm of Salvatore Lo Piccolo's extortion and drug-trafficking empire in the Sicilian capital, Palermo, according to police in Palermo.
The four were responsible for carrying out punishments against businessmen if the extortion fee - or "pizzo" - was not paid. They were identified by police as brothers Nunzio and Domenico Serio, 29 and 28 respectively, Vincenzo Mangione, 28, and Andrea Gioe, 40.
"The ongoing investigation on Lo Piccolo's clan is uncovering a widespread and effective organization," the president of parliament's anti-Mafia commission, Francesco Forgione, was quoted as saying by the ANSA news agency.
"More than anything, it is showing that among those who pay the 'pizzo' - either out of fear or convenience - there are all the strata of the economic and social world in Palermo," Forgione added.
The "pizzo" has long been one of the Mafia's most lucrative sources of cash.
Lo Piccolo was arrested last week after more than a decade on the run. Authorities say Lo Piccolo, who had been sentenced to life in prison, was vying to become the Cosa Nostra crime syndicate's next "boss of bosses," following the arrest last year of Bernardo Provenzano.
Provenzano was nabbed on a farm near Corleone, Sicily, in April 2006 after nearly 40 years on the run.
Elsewhere in Sicily on Tuesday, police were making some 70 arrests of Mafia suspects in the eastern city of Siracusa, the ANSA and Apcom news agencies reported.