Lu Yuyu was found guilty of "picking quarrels and provoking trouble" by a court in the southwest city of Dali, Yunnan province, his lawyer Xiao Yunyang told AFP.
The same charge has been used to prosecute dozens of activists and political dissidents since Chinese President Xi Jinping took office in 2012.
A former migrant worker, Lu and his girlfriend, Li Tingyu, undertook the demanding task of cataloguing labour strikes and other workers' demonstrations across China.
The couple published an exhaustive compilation of these incidents under the alias "Not the News", offering a rare, comprehensive record of how labour tensions were rising.
Lu and Li were arrested in June 2016. Months later, they were awarded the Reporters Without Borders-TV5Monde Press Freedom Prize in the "citizen journalists" category.
AFP was not able to confirm whether Li is still being held in custody.
Xiao told AFP that Lu, who pleaded not guilty, has already filed to appeal his sentence.
"He maintains his innocence," Xiao said, noting that Lu's current mental state was "not bad."
"The four-year sentence of Lu Yuyu is very worrying since he did nothing but compile information that was already publicly available," William Nee, a China researcher at Amnesty International, told AFP.
"Although, ironically, the Chinese government still brands itself as socialist and pro-worker, this harsh sentencing probably indicates that the authorities are willing to crack down on labour rights activists and their supporters."
Working conditions have generally improved over China's more than three-decade economic boom. But many labourers, who often leave their rural homes for menial factory jobs, are still victimised by employers.
According to the Hong Kong-based nonprofit China Labor Bulletin (CLB), there were nearly 2,800 strikes and protests in China in 2015 -- more than in the previous four years combined.
Many of the protests erupted over unpaid wages.
"Lu's sentence is deeply troubling," Keegan Elmer, a CLB researcher, told AFP. "(Lu and Li) provided all of China, officials included, with a glimpse into the reality on the ground."
But "targeting Lu has had absolutely no effect on the ability of workers across the country to do what they have been doing constantly in recent years -- organising online."
Three prominent labour rights lawyers were handed suspended prison sentences in September for "disturbing the social order."
