While Ram Singh, the driver of the bus, Vinay Sharma, an assistant gym instructor and Pawan Gupta, a fruit seller, were arrested from the capital, Singh's brother Mukhesh was nabbed from Rajasthan.
The bus driver along with six others had allegedly gangraped the para-medical student on Sunday night.
The victim is in a critical condition in the Safdarjung Hospital in New Delhi.
On Thursday, one of the four accused was "identified" by the victim's friend during a Test Identification Parade (TIP) at Tihar jail in New Delhi.
Delhi Police will on Friday submit its status report in the high court on its investigations in the case.