Europe's police forces are hunting Bin Laden allies
Three men held in raids in Milan were suspected of trying recruiting people for Osama Bin Laden's Al-Qaeda network, said Italian Interior Minister Claudio Scajola.
"We cannot know if that would have eventually led to a terrorist attack, but we think we can now exclude this possibility."
He said the "subversive cell", which was supported by Bin Laden... had been under surveillance by the security forces for some time.
No further details were given of the fourth arrest, which took place at an undisclosed location in Germany.
One report said a fifth man was being sought in France.
The suspects are said to be of Tunisian, Libyan and Egyptian.
Surveillance
Last April, five suspected militants from Tunisia were arrested in Italy. The new raids are thought to be linked to the continuing inquiry into their activities.
At time of the April arrests, the Italian authorities claimed they had smashed the nerve centre of an Islamic group directly linked to the Al-Qaeda network and to Afghan training camps.
Milan prosecutor Stefano Dambruoso, who is heading the inquiry, has said he expects to bring criminal charges against a number of people by early next year.
The BBC's David Willey in Rome says the Italians have been keeping under surveillance a number of people from Arab countries who are also suspected of using business and financial contacts in Italy to provide logistical support to Islamic militants.
The arrests are the latest in a series of detentions across Europe of terrorist suspects, in the wake of the 11 September attacks in the US.
Terror threat
Italy has been on particularly high alert following the attacks, after a written warning from the US State Department that the country could be targeted by terrorists within the next few weeks.
On Tuesday, four men - thought to be three Libyans and an Algerian - were detained in the Irish Republic under anti-terrorism legislation. They are still being questioned in Dublin.
Two suspects have also been extradited to France - Jamal Beghal, who had been arrested in the United Arab Emirates, and Kamel Daoudi, detained in the UK.
In the UK, an Algerian pilot has been charged with training four of the suicide hijackers who took part in the US attacks.