The group beat and captured James Amburn, their 57-year-old advisor, in June last year after losing about 2.5 million euros (A$3.7 million) in risky investments in the US property market.
The court in Traunstein, southern Germany, found the two men and two women guilty of ambushing him, tying him up, bundling him into the boot of an Audi car and holding him hostage.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The ringleader of the gang, identified only as 74-year-old Roland K., was sentenced to six years in prison for kidnapping and grievous bodily harm, a court spokesman told AFP.
His wife, 80-year-old Sieglinde, received a suspended sentence of 18 months.
Co-conspirator Wilhelm D., 61, will spend four years behind bars, while 64-year-old Iris was handed a suspended sentence of 21 months. Iris' husband Gerhard, who also took part in the kidnapping, was too ill to be judged.
It was a "spectacular attempt at self-justice," the presiding judge said.
In June last year, the two men drove to Amburn's home, seized him, tied him up, and drove him 455 kilometres (285 miles) across Germany.
The kidnapped advisor tried to escape when they let him out of boot but suffered two broken ribs and a shoulder injury during a struggle.
The gang then locked him in a cellar, forcing him to sign papers promising to repay the money they had lost in the investments.
After four days of being held hostage, he managed to hide an encoded SOS in a fax sent to a Swiss colleague, who called the police.
A team of commandos stormed the address, which they worked out from the fax number.
The attackers, who became known in the German media as "pensioner gangsters", gave themselves up without resistance.
The sentences were less than those demanded by prosecutors, who had called for nine years for Roland K. and seven years for Wilhelm D.
At the opening of the trial, Roland K. denied holding Amburn against his will, saying they had invited him "to spend a few days' holiday" in Bavaria, southern Germany.
The two women involved said they regretted their part in the kidnapping.
