He is among 10 activists facing trial for disrupting public order -- a charge that carries a maximum five-year sentence -- after they held banners in public urging authorities to disclose their assets as a check against graft.
The 40-year-old lawyer was a central figure in the New Citizens Movement, a loose network of activists who organised street protests and dinner discussions on causes from educational equality to official corruption.
The United States said it was "deeply disappointed" by the jailing.
"We call on Chinese authorities to release Xu and other political prisoners immediately, cease restrictions on their freedom of movement, and guarantee them the protections and freedoms to which they are entitled under China's international human rights commitments," State Department spokeswoman Jen Psaki said in a statement.
Amnesty International described the verdict as "shameful".
"The Chinese authorities have once again opted for the rule of fear over the rule of law," Roseann Rife, East Asia research director at Amnesty International, said in a statement.
Xu was the first to be sentenced in the case and the other activists are almost certain to be found guilty by China's politically controlled courts.
His jailing comes as new leaders under President Xi Jinping have also carried out a high-profile campaign against graft, warning it could destroy the ruling Communist party.
But the authorities fear any organised dissent that could undermine their control.
A Beijing intermediate court "sentenced Xu Zhiyong to four years in jail" after finding him guilty of "gathering crowds to disrupt public order", it announced on an official account on Sina Weibo, China's version of Twitter.
Xu's lawyer Zhang Qingfang criticised the process as "nothing but a show" in which "the outcome was decided a long time ago".
Describing Xu's response to the verdict, Zhang said he had told the court that it had "pretty much destroyed the last bit of dignity left for China's rule of law".
Zhang added that police had driven him away from the courthouse after the hearing ended, apparently to prevent him from speaking with a group of journalists nearby.
Xu gained prominence as he sought to uphold rights through the court system by offering legal aid in controversial cases -- and was even profiled by China's Esquire magazine in 2009.
But as he increasingly pushed for change, he found himself arrested that year on tax evasion charges. Although these were later dropped, he remained under surveillance and sporadic house arrest.
In total 20 to 40 people involved with the New Citizens Movement have been detained since last year, according to members.
The court in the west of the capital was cordoned off with police tape, while officers in uniform and plain clothes blocked passers-by from approaching.
Xu's prosecution was condemned by international rights groups as well as the US and the EU -- criticism that China's government has rejected as interference in its internal affairs.
His sentence indicated that authorities had little stomach for any amount of protest or organising, said Human Rights Watch researcher Maya Wang, pointing out that Xu pursued limited goals compared to dissidents who sought to overthrow the ruling party.
"On the one hand you can see quite clearly that Xu Zhiyong's organisation across the country threatens authorities. But on the other hand he is among the most moderate among the group of political activists," she said.
"It does show a certain low tolerance by the authorities towards dissent."
The court did not provide additional details about the decision, and searches on Sina Weibo for Xu's name were blocked -- a common occurrence in China's heavily censored Internet for keywords deemed sensitive.
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