Activists, rights lawyers and bloggers are routinely jailed in the one-party state but a new government in place since last year has vigorously pursued detractors.
The four latest arrests were of dissidents who had previously served jail sentences for anti-state convictions. But the current charge they face is much more serious and can carry the death penalty.
Prominent dissidents Pham Van Troi and Nguyen Bac Truyen, freelance writer Truong Minh Duc and Protestant pastor Nguyen Trung Ton were all arrested at their homes on Sunday, their wives told reporters.
In an online statement the Ministry of Public Security said the four were arrested under Article 79 of the criminal code -- trying to "overthrow the people's administration".
They are connected to lawyer Nguyen Van Dai and activist Le Thu Ha, who have already been detained on the same charge.
"My husband fought against social injustice and China's invasion of the East Sea," Troi's wife Nguyen Thi Huyen Trang told AFP, using the Vietnamese name for the South China Sea.
"He did not have any move to try to overthrow the state."
Ton's wife Nguyen Thi Lanh said her husband was already recovering from a recent assault by plainclothes police when he was arrested.
"Voicing support for the people cannot be called trying to overthrow the administration," she told AFP.
Vietnam has competiting claims with China in the South China Sea but Hanoi is extremely sensitive to any criticism about how it handles the issue.
John Sifton, from Human Rights Watch, said 2017 has been "a terrible year" for human rights in Vietnam.
"More cases of government thugs beating up dissidents, longer and longer jail sentences, and now, more arrests," he said.
"Vietnam's allies and donors, especially the EU and Japan, need to speak up," he added.
Last month prominent blogger Nguyen Ngoc Nhu Quynh, known as "Mother Mushroom", was jailed for 10 years for Facebook posts about politics and the environment.
Anti-China activist Tran Thi Nga, 40, was imprisoned for nine years for anti-state activities after a one-day trial last week in the northern province of Ha Nam, to which media access was restricted.
The United Nations human rights office on Friday criticised the intensifying crackdown on rights and the recent convictions.
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