The victims, an 18-year-old girl and three men in their 20s, were shot outside a house on Mission Street just after 1 a.m.
They were running down the street, apparently in an effot to escape the gunman, when they were shot.
All four were taken to University Hospital. The woman was the most seriously hurt, suffering a gunshot wound to the chest. She is now breathing with the help of a ventilator. The three men were struck in the arm, leg and backside.
The gunman, described as a man with his face covered, fled on foot.
Five to nine shots were fired in total outside the home.
The shooting was not believed to be random, and the motive and circumstances remain under investigation.
Tuesday night, the mayor held a meeting and vowed to boost the police presence in Montclair.