The four legislators in question -- Nathan Law, Lau Siu Lai, Edward Yiu and Leung Kwok-hung, known among locals as "Long Hair" -- were elected to the Hong Kong Legislative Council, known locally as LegCo, last year.
All four used their oaths of office to protest attempts by Beijing to exert more control over internal affairs in Hong Kong, which has been an increasing concern of citizens in the city since the Chinese took control 20 years ago.
Though part of China, Hong Kong is a Special Administrative Region that enjoys far more freedom than cities in the mainland.
But locals worry that China is clamping down on this bustling metropolis of more than 7 million, violating the governing principle of "one country, two systems."
All four disqualified lawmakers were involved in the Occupy movement of 2014, when thousands took to the streets to demand democratic reforms --- including the unsuccessful demand that the city's leader, the Chief Executive, be directly elected by the people of Hong Kong.
Democracy activists had hoped the more subdued swearing-in protest meant the lawmakers wouldn't be penalized.
Their ejection robs the pro-democracy movement of its majority in the portion of Hong Kong's legislature that's democratically elected (some seats are appointed), and its ability to wield veto power over some key legislation.
That effectively changes the balance of power in the entire legislature, according to Antony Dapiran, a Hong Kong-based lawyer and author who's just written a book about dissent in the city.
"It's pretty bad for rule of law in Hong Kong when you see the courts being used to kick out democratic lawmakers like this," Dapiran told CNN. "Hong Kong can no longer pretend to (be) any kind of democracy."
Demosisto, the political party which counts Law and activist Joshua Wong among its members, called the decision the "worst assault on our democracy."
"More than 180,000 voters had their voices silenced in the legislative body. Demosisto condemns the manifest interference of the Beijing government to cripple Hong Kong's legislative power," the group said in a statement. "In light of manipulation of election results by Beijing, it is more important than ever for Hong Kong to stay strong and firm against the autocracy."
About 2.2 million people voted in the September 2016 elections,
according to the Electoral Affairs Commission, with a turnout of 58% -- up from 53% in 2012. Hong Kong does not allow postal voting or early voting.
Law -- at 23, the youngest person ever elected to be elected to LegCo -- Lau, Yiu and Leung vowed to appeal any decision to remove them from office.
Six lawmakers used their oaths of office to protest Chinese authoritarianism last year.
Two lawmakers who are advocates of Hong Kong's independence from China -- Yau Wai-ching and Sixtus "Baggio" Leung -- were ousted after swearing and insulting China during their oaths.
While Law, Yau, Yiu and Leung's oaths were initially accepted by the LegCo president, Yau and Baggio Leung's were not.
The pair was due to retake their oaths, but Hong Kong government officials sued to prevent them being sworn-in again, arguing they had forgone their opportunity.
Before the court could rule, the Chinese central government employed its rarely-used power to re-interpret Hong Kong's constitution to bar Yau and Leung from being sworn in.
That decision opened the door for Beijing to use the courts to influence Hong Kong's elections, using minor infractions as a way to eliminate opposition lawmakers, said Dapiran.
"It effectively created a new law from scratch, instituting new legal requirements that weren't there before," he said. "Beijing is effectively dictating who is allowed to run and who is allowed to be elected, and that's no longer a democracy."