Police said the four turned themselves in to authorities, accompanied by their lawyers, and were being questioned Friday.
Photos of suspects had already been plastered in newspapers and on the Internet, as police released images sent by self-described eyewitnesses.
"Thank you all for your help!" the Montreal police said on its Twitter page.
Smoke-bomb attacks on several metro stops caused a shutdown at the height of rush hour the previous day, creating a frustrating and traffic-clogged commute for many thousands of Montrealers.
There were media reports that the attacks were being tied to hardcore anti-tuition students, who consider the main protest groups too timid and not far-reaching enough in their goals.
A merchants' association and a conservative economic think-tank have estimated that the subway incident alone cost Montreal several million dollars in lost productivity.
According to the Board of Trade of Metropolitan Montreal, the 90-minute delay caused Thursday by smoke bombs in several stations cost the city $11 million. The Montreal Economic Institute came up with its own figure of $9.3 million.
The ongoing student unrest has even prompted a security message from the U.S. government.
American tourists were warned of possible "unforeseen violence," "vandalism" and "arrests," in a message issued Apr. 27 by the U.S. embassy in Ottawa to visitors heading to Montreal.
The message warned Americans traveling to Montreal that although businesses remain open, traffic and public transportation may be disrupted. They were also told they could run into demonstrations and, while most of them have been peaceful, some might turn "potentially violent."
News of the American message, which had largely gone unnoticed, spread in the local media Friday and fuelled speculation about what impact the unrest might have on the local economy.
Such fears have been intensifying now that the summer festival season is approaching. Montreal's famous comedy and jazz festivals, and Formula One parties, tend to be concentrated near the streets and public squares that have been awash in frequent protests.
One student association, comprising arts majors at UQAM university, decried this week that the protest movement had actually been too peaceful and it promised to disrupt Grand Prix parties.
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