These local arrests occurred just two days after Hong Kong offered rewards for eight pro-democracy activists currently residing abroad.
The announcement of these bounties has sparked widespread international criticism, following the implementation of a comprehensive national security law imposed by Beijing.
Demosisto was co-founded by activist Nathan Law, who now resides in the United Kingdom and is one of the primary targets of this week's announced rewards.
Law recently spoke to the BBC, expressing the immediate increase in personal danger he now faces.
The eight individuals subject to bounties of HK$1 million (£100,581; $127,637) are based in the United Kingdom, the United States, and Australia—countries without extradition treaties with China.
Hong Kong, operating as a special administrative region of China, affords its people broader freedoms under Beijing's "one country, two systems" policy in comparison to mainland China.
Among the four individuals arrested is Ivan Lam, the former chairman of the disbanded political party Demosisto, according to local media reports. The four men arrested on Wednesday night face allegations of "conspiracy to collude with a foreign country or external elements to endanger national security" and "conspiracy to commit acts with seditious intent."
If found guilty, their offenses carry a potential life imprisonment sentence under the national security law.
Additionally, on Wednesday, law enforcement seized banners and flags associated with the online shopping app Mee, which serves as a resource for Hong Kong residents seeking information about pro-democracy supporting restaurants, shops, and service providers.