Leung was a prominent independence activist and the spokesman of Hong Kong Indigenous, a pro-independence group in the city that was outspoken about "localism" and the need to preserve a distinct Hong Kong identity.
In 2018, the 30-year-old activist was convicted of assaulting a police officer and rioting during what is now known as the Fishball Revolution.
The unrest began when authorities attempted to crack down on unlicensed hawkers selling street food during the 2016 Lunar New Year holidays in Mong Kok, but clashed with protesters who opposed their actions as an attack on local traditions.
Initially sentenced to six years, Leung had his sentence reduced by two years for good behaviour, according to local media reports.
Leung's release comes during a crackdown on political dissent in Hong Kong, with authorities arresting a majority of Hong Kong's outspoken pro-democracy activists over the past two years.
Many of the city's prominent activists are currently behind bars or have fled overseas to continue their activism.
In a statement posted on his Facebook page early on Wednesday morning, Leung said he had been released from prison and is back with his family.
"As required by law, I am subject to a supervision order upon release," he wrote in the post, adding that he would stop using social media and will not be taking any media interviews or visits.
"After four years, I want to cherish this precious time to reunite with my family and resume a normal life with them," Leung said, before thanking his supporters for their concern and love.
He is known for coining the slogan "Liberate Hong Kong, Revolution of our Times" for his election campaign, when he attempted to run for a seat in the legislature in 2016. He was later disqualified.
The phrase later became a popular protest slogan during the 2019 protests, but authorities have since banned it, stating that it has secessionist connotations that are illegal under the national security law that was implemented in 2020.
The law outlaws secession, subversion, terrorism and foreign collusion to intervene in the city's affairs.
Leung advocated so-called forceful resistance against political violence in his campaigns, which was considered a polarising opinion and drew opposition from the city's more traditional pro-democracy camp.
However, his stance of a more active form of resistance also drew the attention of young voters, and many of his ideas, such as "leaderless" protests, were later employed during the months of anti-government demonstrations in 2019.
