"Three men and one woman were admitted late last week and early this week in our hospital with a bacterial infection from diseased pigs," said Nguyen Hong Ha, deputy director of the Tropical Diseases Institute in Hanoi.
"They had high fevers, headaches and vomiting ... After tests, we concluded that all four showed symptoms of the meningitis form of the disease," he said, adding that their lives were not considered to be in danger.
The bacterial disease, streptococcus suis, infects people who handle raw meat or eat undercooked pork dishes.
Ha said relatives of the patient who was in the most serious condition said he had recently eaten pig's blood pudding, a traditional dish.
Twenty-six people had now been admitted to the Hanoi hospital, and two of them had died of the disease, he said. A third human death reported by state media earlier this week has so far not been confirmed by laboratory tests.
Twenty people have also been treated for the disease in southern Vietnam this year.
"The streptococcus suis is a worldwide disease in pigs, but it's more common where there are high densities of pigs, like in Vietnam and China," said UN Food and Agriculture Organisation (FAO) animal health expert Jeffrey Gilbert.
"The concern with the streptococcus suis at the moment in Vietnam is that there seems to be an increase in human cases compared to previous years."
Meanwhile, an outbreak of the "blue ear" swine disease, or Porcine Reproductive and Respiratory Syndrome (PRRS), has spread through Vietnam's three central provinces of Quang Nam, Quang Ngai and Danang city.
The FAO's Gilbert said it was so far unclear whether there was a link between the human cases of streptococcus suis and the PRRS outbreak.
"Potentially diseases can co-exist," Gilbert said. "If one disease affects the immunity of the animal it will be more likely to get another disease."