The men, three Jordanians and an Algerian, are said to have belonged to or supported al-Tawhid, whose operational head is thought to be alleged senior al-Qaida figure Abu Musab al-Zarqawi.
Zarqawi is a Jordanian suspected by Washington and Berlin of ties to al-Qaida and to have been a link between the group and the Iraqi regime under Saddam Hussein.
According to prosecutors on Tuesday, the Jordanians on trial in Duesseldorf, western Germany, were members of a German branch of al-Tawhid which plotted to attack Jewish targets in Duesseldorf and in the capital Berlin.
The Algerian is said to have supported them.
Prosecutor Dirk Fernholz said that al-Tawhid "is linked to an international network of Islamists prepared to use violence."
The accused are Muhammad Abu Dhees, 39, Ismail Shalabi, 30, and Ashraf al-Dagma, 34, all Jordanians charged with membership of a "terrorist organisation" and falsifying documents; and Algerian Djamil Mustafa, 30, who is accused of supporting a "terrorist organisation".
Another man, 27-year-old Jordanian Shadi Muhammad Mustafa Abd Allah was jailed for four years by the same court in Duesseldorf on similar charges.
His relatively mild sentence reflected the insight that he gave
authorities probing Islamic extremism and operations in Germany and elsewhere.
Tuesday's trial began as Germany's Tagesspiegel daily reported that German officials were studying an audio tape apparently recorded by al-Zarqawi in which he calls for the destruction of the United States.
A memo uncovered in Baghdad and linked to al-Zarqawi, said to have detailed plans to spark communal violence between Iraq's Sunni and Shia Muslims.
US Secretary of State Colin Powell claimed on Monday that the memo gave "some credence" to US pre-war claims about Saddam Hussein's links with al-Qaida.