Police believe the two girls and two boys caught a taxi to Bridge Inn Road in Wollert, north of Melbourne, about 3am on Tuesday and told the driver that a relative was coming to pay the $45 fare.

As the teens went to walk away, the driver, Keilor Downs man Akbar Ali, attempted to restrain one of the boys.

The other three teenagers are then alleged to have unleashed a violent assault on the 25-year-old.

Police said they repeatedly punched and kicked the driver, inflicting serious head injuries and shattering two of his teeth.

Mr Ali's face was also left bruised and battered after he suffered a broken nose and cuts down his left cheek.

Mr Ali told Channel Seven the group of teens also began racially abusing him.

"(They said) You black people come here and why are you not going back to your countries," he said.

Mr Ali said he feared for his life during the incident.

"I just told them, 'don't do this'," he told Channel Nine outside the Northern Hospital on Tuesday afternoon.

It is believed that during the alleged attack, Mr Ali was on a conference call to his friend Musman Muhammad.

"I only heard screaming sounds??? I heard him crying out," Mr Muhammad told Channel Nine.

"It was really awful."

Police said two witnesses attempted to help the driver and contacted emergency services.

The alleged assault was still taking place when police arrived.

Police arrested a 17-year-old Carlton girl, a 16-year-old Mill Park girl, a 15-year-old Coburg boy and a 13-year-old boy from Reservoir.

The four have been interviewed and are expected to face charges including affray, recklessly cause injury, taxi-fare evasion and assault, police said.

Mr Ali was released from hospital on Tuesday afternoon and said he planned to return to work next week.