The four, starting in the 1990s, allegedly tempted collectors with a cover story that up to 50 paintings lost under the Nazi seizure of a Jewish art collection, and known only from old photographs, had been rediscovered in private homes, after reclusive art lovers snapped them up half a century ago.
With the art market in overdrive, museums and millionaire collectors in Cologne, Germany's capital of modern art, lined up to buy the canvasses by early 20th century German artists.
In the biggest sale, Red Picture with Horses, supposedly painted by Heinrich Campendonk (1889-1957) of the Netherlands, sold at auction in Cologne for 2.4 million euros (3.4 million dollars). It was the highest price ever paid for work by the Rhineland expressionist.
Other works, seemingly by German luminaries Max Pechstein, Max Ernst, Andre Derain, Kees van Dongen and Fernand Leger, were sold to museums in Germany and the Netherlands and in Paris.
Late last year, police arrested three suspects who are now in custody, after a tip off that the labels on the back of the paintings were fake.
Helene B, her husband Wolfgang B and her sister Jeanette S had claimed the paintings came from a family collection begun by the sisters' grandfather, industrialist Werner Jaegers, who died in 1992.
The surnames of the accused have been withheld under privacy guidelines for the German media.
Jaegers, the group claimed, had purchased the works from Alfred Flechtheim (1878-1937), a noted pre-war Jewish art dealer in Cologne whose collection was broken up and sold cheaply under the Nazis.
The fourth accused, Otto S, 67, offered paintings which he claimed he had inherited from his own grandfather, Wilhelm Knops, a master tailor who died in 1957.
The two collectors had been friends and quietly picked up treasures without anyone noticing they were art lovers, so the story went.
Prosecutors say Wolfgang B was the one who really painted all the pictures, with Helene B and her sister passing them off as genuine at auction houses. The women spaced out the sales so no suspicion was aroused, whilst also offering genuine originals from time to time.
'I had a high impression of them,' admitted Henrik Hanstein, owner of the dealer Lempertz, in an interview with the June issue of the magazine Art.
Prosecutors say the group was careful to only offer paintings which were known to have vanished without trace decades ago, but had been described, or photographed, in catalogues and other records.
Rediscovering art treasures that are believed to have been lost in bombing and looting during the Second World War is one of the great dreams of professional art hunters in Germany.
Three years ago, suspicions were triggered by the forged Flechtheim Gallery labels on the back of the Jaegers paintings.
Ralph Jentsch, an expert on the Jewish dealer, was able to demonstrate the labels were nothing like the ones Flechtheim really used. The provenance inquiry led to the paintings being unmasked.
The tally of those who appear to have been fooled was a who's who of dealer consultants and museum curators.
So far, 14 Jaegers paintings have been seized by police and submitted to laboratory tests. Two pigments, a titanium white and a blue, had not been on the market when the pictures were allegedly painted. Another 33 paintings are to be tested.
Buyers are now seeking damages from the Lempertz dealer gallery for some of the forgeries.
The massive prices paid for some art works are a big temptation for white-collar criminals. Whether it is Chinese terracotta figures, Art Nouveau glassware, Picasso paintings or Giacometti sculptures, there is a market waiting for forged works.
A dealer from Mainz, Germany was sentenced last month to seven years' jail for passing off fake bronzes he attributed to Swiss sculptor Alberto Giacometti (1901-66).
Hanstein told Art magazine that Lempertz would in future use X-ray machines to check out art before it was sold. The company's researchers, restorers and laboratory will cooperate to foil forgers.
Faking techniques are bound to get more sophisticated.
'Our job is to keep one jump ahead of the rogues,' he vowed.