Nguyen Khac Toan, who described prison as "like hell", said he was freed on Tuesday as part of an amnesty before the Lunar New Year which starts on 29 January.
"It's good that I am free but I still have to be under house arrest for three more years," Toan, a businessman and ex-officer in the North Vietnamese army, said on Thursday.
He was convicted in December 2002 of having collected complaints, information and documents and "illegal" articles put out by opposition figures, and of having emailed information to some exiled Vietnamese groups.
"After four years in jail, I feel very weak physically because prison is like hell. We had to work, there were so many prisoners in jail and we had to obey very tough regulations," said Toan, 50.
Human rights groups had criticised Vietnam over Toan's arrest.
"After four years in jail, I feel very weak physically because prison is like hell"
Nguyen Khac Toan
"When I was arrested prior to the trial, the authorities accused me of carrying out 'propaganda against the socialist state'. But later, I was charged with being a spy," Toan said.
Vietnam retains a tight grip on all domestic media activities.
No private outlets are permitted and the regime is consistently accused by human rights organisations of cracking down on dissent and jailing anyone criticising the Communist Party and the government.
"I was part of a movement asking for moderate democracy to reform the Vietnamese system from dogmatic socialism to a more open, multi-party society," Toan said.
In jail, he said, he was put in a cell with 65 others held on political and national security charges. They included some members of ethnic minorities.
The prisoners had to work long hours making bamboo products for export and were barred from talking to anyone in other cells.
They were confined to the cells for more than 14 hours a day and only allowed out in the front yards for three days a year, during the spring festival.
Nguyen was charged with sending
emails to Vietnamese exiles
"Now, it's difficult for me to move around... I have injury in my backbone and problems with blood pressure," Toan said.
He was freed early for good behaviour but will be mostly confined to his home.
Toan said he was ordered "not to resume relations" with other prominent democracy activists.
"I am not allowed to keep and popularise documents on democracy," he said, adding he must report to the police every month and is barred from giving interviews.
Some years ago, Toan's mother too incurred the wrath of the authorities by signing petitions with the families of other dissidents which urged the government to respect human rights.