All four cases are imported.
The Myanmar sailors, aged 40 and 31, traveled to northern Quang Ninh Province last month.
The other two are Hanoian men aged 56 and 35, who returned from Bangladesh via a repatriation flight on July 3.
All four received treatment at the National Hospital for Tropical Diseases in Hanoi, where they will remain for 14-day monitoring.
Vietnam has recorded 509 Covid-19 patients, of whom 373 have recovered.
Keeping its community virus-free for over three months, the nation is dealing with a new Covid-19 wave following a case detected in Da Nang on July 25.
A total of 93 domestic infections have been reported since, 79 of them in Da Nang and the rest in Hanoi, HCMC, Quang Nam and Quang Ngai in the central region, and Dak Lak in the Central Highlands.
Early Friday, Vietnam reported 45 new cases, the largest number of domestic infections reported in one day since it detected the very first cases in February.