ZURICH - One of four paintings stolen at gunpoint from a Swiss museum four years ago in a stunning art heist has been returned to Zurich, the city's senior public prosecutor said on Friday, meaning all the paintings have now been recovered. The painting by Edgar Degas, regarded as one of the founders of Impressionism, was returned several months ago, but the information was withheld to avoid compromising investigations while Swiss and Serbian police were still hunting a painting by Paul Cezanne, still missing at the time. Serbian police recovered this last painting, Paul Cezanne's "Boy in a Red Waistcoat" earlier in April. Worth a reported $109 million, the Cezanne masterpiece was the best known painting in the estimated $163 million robbery. Two of the stolen paintings, a 1879 Monet and a 1890 Van Gogh, had been found just a week after the theft when police discovered the getaway car in the car park of a Zurich psychiatric hospital. "Due to a trace found in the getaway vehicle, investigations led to a person in Serbia who had already committed offences in Switzerland in the past," the Zurich state prosecutor's office said at a press conference. "The nature of the offences and the modus operandi, as well as potential accomplices suggested that the perpetrators were very dangerous criminals." The Swiss and Serbian authorities then launched a vast operation codenamed "Waistcoat", involving up to 30 investigators from six countries with a whole network of undercover agents monitoring phones and tracking movements. A total of 2.8 million euros ($3.7 million) were handed over to lure the thieves in a "sting" operation. Four men were arrested in Serbia in connection with the theft, the Zurich state prosecutor's office said. According to Swiss tabloid Blick, one of the four had been a contestant on Serbia's version of "Who wants to be a Millionaire" where he had pocketed 80,000 Serbian dinars ($940).