The Delhi Police registered a case under the Protection of Children from Sexual Offences (POCSO) Act against the boy after the girl's mother lodged a complaint.
"The Indian Penal Code (IPC) provides children below seven years of age certain protections against prosecution".
The two children, who are kindergarten students of a private school in Dwarka, were referred for counseling, the police said on Thursday.
According to the mother, the child told her that a boy from her class opened her trousers and touched her undergarments. The police filed the case on the complaint of the girl's mother.
On Friday, the victim returned home from her school and complained of pain in her private parts.
The assault caused wounds in the child's private parts, her mother said. "We are examining those provisions, even as we are handling the matter with utmost sensitivity", Delhi Police spokesperson Dependra Pathak told mediapersons. A medical-legal was made first and later a police case was registered against the four-year-old boy at the Dwarka south police station. The girl was then taken to a hospital where doctors confirmed the sexual assault.
In the complaint, the mother also said, "I spoke to the principal as she had not bothered to answer my call or respond to my mail".
"She tried to push him but to no avail as there were no other children in the classroom and there was no staff around", the statement stated.
The girl also told them that when the incident took place no staff member was present inside the class.
She further added that there was no ayah or teacher in the washroom or classroom at the time of the alleged assault.
She said her daughter's delayed exit from the classroom, as she saw in CCTV footage, corroborated the sexual assault allegation.
While, NDTV reported that the school has said that it was "impossible" that 4-year-old girl was sexually assaulted by another classmate of the same age, their lawyer had earlier denied charges of negligence. "Absolutely not", says Dr Samir Parikh, the director of Fortis Healthcare's department of mental health and behavioural sciences.