Wives said the four activists were all arrested Sunday at their homes on the same charge leveled at lawyer Nguyen Van Dai and activist Le Thu Ha, who have been held since December 2015.
Dai is the founder of a group advocating full rights throughout Vietnam, Brotherhood for Democracy. From 2007 he served four years in jail.
Wife spurns charges
Those arrested Sunday were named as Pham Van Troi, Nguyen Bac Truyen, freelance writer Truong Minh Duc and Protestant pastor Nguyen Trung Ton.
"Voicing support for the people cannot be called trying to overthrow the administration," said Ton's wife, Nguyen Thi Lanh, who added that her husband was still recovering from being assaulted by plainclothes police.
John Sifton, from Human Rights Watch, said 2017 has been a "terrible year" for human rights in Vietnam.
He urged Vietnam's allies and donors, especially the EU and Japan, to "speak up" more loudly.
Last Friday, the UN human rights office criticized the intensified crackdown and recent convictions.
Last month, Nguyen Ngoc Nhu Quynh, a prominent blogger known under her pseudonym,"Mother Mushroom," was jailed for 10 years for social media posts about politics and the environment.
Last week, anti-China activist Tran Thi Nga, 40, was jailed for nine years for alleged anti-state activities after a brief trial to which media access was restricted.
On Wednesday, the US embassy called on Vietnam's government to release all prisoners of conscience, notably Tran Thi Nga, and to allow freedom for peaceful speech and assembly "without fear of retribution."
Reporters Without Borders ranks Vietnam on press freedoms down at 175 on its list of 180 countries.
ipj/tj (AFP, dpa)