The latest victim died in a hospital in northwestern Xinjiang region, Xinhua reported. The other three babies died of kidney failure in northwestern Gansu and eastern Zhejiang provinces.
Health officials say 6,244 infants were sickened by the formula tainted with the chemical melamine. About 1,300 of them remained hospitalized suffering malnutrition, kidney stones or acute renal failure.
The report said dairy giant Sanlu in Hebei province in north China was the first company to be exposed by the scandal and its chairwoman has been detained by police for questioning.
Since then inspectors found melamine in 69 batches of baby milk powder produced by 22 other companies.
CNN reported authorities arrested 12 more suspects, bringing to 18 the number of people detained.
Among those arrested, 12 were accused of being involved in the collection of milk and production of milk products, while the others were accused of illegally selling the chemical.
Melamine is strictly forbidden in food processing, but earlier Xinhua reports quoted experts as saying it is added to raw milk to make its protein content appear higher.