No arrests had been made until Monday night, the police said.
"The four men are described as wearing dark clothing and long black jackets. They all had long beards. One of the men is described as younger and slimmer than the other three. They all fled in the direction of Oxford Street," the statement said.
"Detectives are keen to speak to anyone who was in the area at the time or who may have information about the incident. In particular they want to speak to those people who assisted the couple at the scene following the attack," it added.
According to Brar, a mobile phone had been recovered from the scene, which may help lead the police to the assailants.
The police were also expected to go through CCTV footage from the area that has several cameras.