A court in the city of Duesseldorf also convicted the men of belonging to a foreign terrorist organization, and sentenced them to between five and 12 years in jail for the plot in September 2007.
The three Germans and one Turk were in at an advanced stage of planning to use three car bombs with 250 kg of explosives each to attack US targets in Germany to kill hundreds of people. They were known as the "Sauerland group" after the area of western Germany where they were caught.
Police had been watching their rented holiday cottage for several months before swooping in to make the arrests.
"You were planning a monstrous bloodbath that would have killed an unfathomable number of people," said Judge Ottmar Breidling at the conclusion of the 10-month trial. Their goal was to kill at least 150 American soldiers stationed in Germany, he added.
"Never before had there been a terror attack of that dimension in Germany," Breidling said. "You were blinded by a strange, hate-filled notion of jihad and you turned yourselves into angels of death in the name of Islam." The four wanted to time their car bomb attacks to coincide with a vote in Parliament on whether Germany should extend its military presence in Afghanistan. They wanted to influence public opinion in Germany against the Afghan mission.
Breidling sentenced the group's ringleader Fritz Gelowicz to 12 years in jail. He also handed Daniel Schneider a 12-year sentence because he fired at a police officer during an escape attempt.
The court also sentenced Adem Yilmaz, a Turkish citizen, to 11 years and Atilla Selek, a German citizen of Turkish origin, to five years. Prosecutors sought terms of 5-1/2 to 13 years.
All four, who were members of the radical Islamic Jihad Union, reacted without any visible emotion to the verdict.
The four had identified discos and airports, even the US Ramstein air base, as possible targets. They had faced a maximum of 15 years in jail. Confessions and their cooperation with prosecutors reduced their prison sentences, the judge said.
"The confessions have provided an extraordinary glimpse into the structures of Islamic terrorism," the judge said.
The CIA helped to foil the plot after months of close cooperation with German authorities, German media have reported.
Some 600 German investigators began monitoring the group around the clock in early 2007 and arrested them on Sept. 4 that year.
Germany has been largely spared terror attacks by militants, although the leaders of the 9/11 attacks on the United States in 2001 were students based in Germany.
