Italy has mounted a large anti-terrorism campaign
The prosecution alleged during the trial that they were connected with the al-Qaeda network.
They were convicted of association with intent to obtain and transport explosives and chemicals.
Two were sentenced to five years in prison and the other two received four years. All four denied the charges.
One of the men, Essid Sami Ben Khemais, who was alleged to be the group's leader, was suspected of planning an attack on the US Embassy in Rome in January last year.
But none of the men was thought to be connected with the 11 September attacks in the US.
No explosives were found in the men's possession, but prosecutors said that recordings of telephone conversations indicated a conspiracy to obtain them.
The group's leader has apparently admitted to trafficking in false documents, but only for personal profit.
Ben Khemais had already been tried in his absence to 20 years' imprisonment in Tunisia, where a military court found him guilty of membership of a terrorist organisation operating overseas.
His lawyer had denied the charge, saying he was simply a member of the Tunisian opposition.
The four men were arrested between April and October last year as part of a joint Italian and German investigation.
The BBC's Jonny Dymond, reporting from Rome, says the relatively light sentences in Italy were because of two factors.
One was a deal between the defence and the presiding judge to allow a shorter trial in return for some leniency - and the other was the fact that the men were charged under laws that were changed after 11 September.