The woman was admitted to hospital in Delhi after an attack in the northern state of Uttar Pradesh in September.
She later died of her injuries.
The handling of the investigation was widely criticised, with allegations that local police had deliberately mismanaged the case.
The case was later transferred to India's Central Bureau of Investigation.
The four suspects, who all belonged to a higher caste, allegedly attacked and raped the victim in a field in Hathras district. She died two weeks later.