For dealer James Cohan , this year’s Venice Biennale is both a breakthrough moment and a stress test. The respected player in the market’s middle tier represents four artists in the 61st International Art Exhibition, which opens to the public on Saturday, after four days of previews. Three of them— Ranti Bam , Tuan Andrew Nguyen , and Kennedy Yanko —are showing their own work. The fourth, Yinka Shonibare , is the founder of Guest Artists Space Foundation (G.A.S.), a Nigerian non-profit residency that is among the 110 artists and collectives invited by the late curator Koyo Kouoh to participate in the biennale’s central exhibition, “In Minor Keys.” “We’re thrilled to have our artists represented,” said Cohan, who founded his New York gallery with his wife Jane in 1999. “It’s a whole set of eyes that go to Venice that don’t go to fairs in the U.S. or exhibitions like the Whitney Biennial .” The newly restored facade of the Central Pavilion of the Biennale in March. Photo: Stefano Mazzola/Getty Images.…