The four, who lived near Bologna, allegedly used the internet to "circulate jihadist propaganda, manuals on carrying out attacks and songs celebrating martyrdom," La Stampa reported.
One of the documents reportedly discussed how to carry out an attack at the European Central bank.
Interior Minister Angelino Alfano said the four had been under surveillance for a while.
"For various reasons, [the four] joined and committed themselves to the spread of violent extremism," Alfano said.
"These results show that our security systems work and that, through prevention, we aim to reduce the risk of an attack."
The move came after Italy raised its terror alert in the wake of the Paris attacks, in which 130 people died, and amid a tip-off from the FBI that attacks were being planned at key monuments in Rome and Milan. Foreign Minister Paolo Gentiloni said last Thursday that Italy was hunting five jihadists in relation to the warning.
Earlier this month, Italian police announced a swoop on a European jihadist network that was allegedly planning to try to spring its leader out of detention in Norway.
Seventeen people were targeted in the raids across Europe - 16 Kurds and a Kosovan. Six of them were arrested in Italy, four in Britain and three in Norway.
Investigators said the network was trying to free Norway-based fundamentalist preacher Najmuddin Ahmad Faraj - also known as Mullah Kreka - who is listed as a terrorist by the United States and United Nations.
Among those arrested in Italy was Abdul Rahman Nauroz, who police said was “particularly active in recruitment activities” with his apartment in the northern city of Merano being used as a “place for secret meetings and a crossroads for aspiring jihadists”.