The farm ministry will inspect all cows born between March and April 1996 for symptoms of mad cow disease, Agriculture, Forestry and Fisheries Minister Tsutomu Takebe said Friday.
The four infected cows so far discovered in Japan were all born during that period.
The ministry hopes to begin the checks for mad cow disease, formally known as bovine spongiform encephalopathy, in June. Some 26,000 cows will be examined at livestock health centers in every prefecture, Takebe said.