Jean Pascal Boucher, a spokesman for the director of criminal and penal prosecutions, says the Crown decided there was insufficient evidence to support the charge.
Three woman face charges of mischief and conspiracy while a man faces an additional charge of possession of a prohibited weapon.
The four accused are free on bail pending their next court appearance on Sept. 3.
At least three smoke bombs were tossed into several subway stations in Montreal on May 10, 2012, shutting down the entire system for hours during the key morning commute period.
Media reports at the time tied the attacks to hardcore student protesters who considered the main protest groups too timid and not far-reaching enough in their goals.