The formal arrest of one couple aged 74 and 79, another aged 63 and 66, plus a 60-year-old American, came after police commandos stormed the cellar of a property in southern Germany at dawn on Saturday to find the kidnapped 56-year-old man.
Police said they believed the man, who was reported missing last week in Speyer near Heidelberg, was ambushed and assaulted by the American and the 79-year-old, bound up with tape and bundled into the boot of a car.
He was then driven more than 450 kilometres (280 miles) early Wednesday to Chieming near the Austrian border and imprisoned in a cellar for four days.
"He was obviously beaten as well, sustaining bruises and two broken ribs," Chieming police said in a statement. "An attempt to escape during a 'cigarette break' went wrong."
Investigators said they believed that demands for money were behind the kidnapping, and that allowing the 56-year-old to make a phone call in this regard proved to be the gang's undoing.
"The man managed to alert the person who we suspect tipped us off as to his precarious situation and his whereabouts. This was the crucial point for the police operation and its successful outcome," they said.