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.
The victim's family accused local police of cremating her body without their consent, although the authorities denied this. A senior state official also appeared to imply the woman was not raped because semen was not found.
Dalits, formerly known as untouchables, face widespread discrimination and even violence in India, despite laws to protect them.
Rape and sexual violence have been under the spotlight in India since the 2012 Delhi bus rape and murder case, which led to huge protests and changes to the country's rape laws. But there has been little sign of crimes against women and girls abating.