All of the four new patients had come into close contact with COVID-19 cases, the ministry said in its 18.00hrs update.
The outbreak has been brought under control in Hai Duong as only several cases have been detected these days as opposed to dozens a week ago.
Hai Duong has taken drastic anti-coronavirus measures, including a two-week social distancing order, starting on February 16.
The virus recurred in Vietnam less than a month ago and has spread to 13 cities and provinces across the country.
As many as 831 new infections have been since confirmed, including 647 cases in Hai Duong alone.
The same day, 35 COVID-19 patients were discharged from hospital after making a full recovery from the disease. Of them, 27 are in Hai Duong province.
Also on February 26 the Ministry of Health announced another imported case returning to Vietnam from overseas.
The 24-year-old Vietnamese patient was said to have illegally crossed the land border into the Mekong Delta province of Dong Thap on February 23.
She tested positive for the SARS-CoV-2 virus two days later.
Among active cases, an elderly patient in Da Nang city, central Vietnam, is in critical condition and has been relying on ECMO, a sophisticated therapy to save patients in critical condition. The patient has suffered from hypertension and diabetes for years.