The group of four had shared thousands of posts, photos, and videos in support of extremist ideologies, and expressed praise for terror attacks claimed by Isis.
The suspects, who were based in Milan and Perugia in central Italy, have been charged for incitement to murder and using an electronic network for terrorist purposes.
One of the suspects was already being held in a Milan prison after a December arrest for drug dealing, and the remaining three have now been arrested.
Police began the investigation in 2015 after uncovering several Facebook pages which appeared to have links to Italy and were used to express support for Islamic extremist views. Local police teams were assisted by anti-terror unit Digos and the Italian postal police.
One of the suspects was a Tunisian citizen legally resident in Italy and who occasionally worked as a pizza chef, however the others were undocumented, homeless, and involved in drug dealing, police said.
Investigations revealed that all four had multiple Facebook profiles and used wireless networks to ensure anonymity.
Thursday's announcement came as Interior Minister Marco Minniti said Italy would be increasing security measures in crowded areas, following an attack in London which left four dead including the attacker.
Minniti chaired an extraordinary meeting of anti-terror unit CASA on Thursday morning, to evaluate the terror threat in Italy and the security measures in place. The terror threat level in Italy is currently at level 2, the highest possible in the absence of a direct attack.