Japan's Okayama prefecture Friday started culling around 200,000 chickens at a farm after a fourth bird flu outbreak, a media report said.The case was reported to the local livestock health centre by the farm Thursday, after 28 chickens were found dead Wednesday, Xinhua reported.Of 28 dead chickens, four tested positive for the flu and a further genetic test confirmed the virus.The government has imposed a ban on the movement of chickens and eggs at six farms within a three-km radius of the farm where the chickens died.Fifteen other farms in a 10-km radius have also been ordered not to ship their products.This is the fourth case of avian influenza detected in Japan this winter. The previous three cases, two in Miyazaki prefecture and one in Yamaguchi prefecture, have resulted in the culling of thousands of chickens.Okayama is the fourth largest chicken egg producing prefecture in Japan.