A relative told CBC News that three men, aged 18 to 20, and a girl, 17, were out celebrating one of their birthdays before the crash about 1 a.m. They were all from the surrounding area.
RCMP said their bodies were found inside the vehicle just after 3 p.m.
The car was driving up a hill after midnight when it slid off the icy road and into the river in the backwoods near Tracadie-Sheila on the Acadian Peninsula, police said.
Divers searched the murky water all day before it was finally pulled from the water.
The names of the victims are not being released until their families are notified.
On Facebook people are encouraging fellow Tracadie-Sheila residents to turn off their Christmas lights as a sign of respect for the victims.