Police were called out to the complex on Laura Koppe near Etta in northeast Houston around 11:40pm.
According to police, the boy climbed onto a tall furniture chest and obtained a handgun hidden there. It was later on, with a two-year-old child in the room, that the four-year-old fired the gun. Police say the boy was struck once in the front of the head.
"The child was supposed to be asleep in the room," said Officer Brian Evans with the Houston Police Department. "The parents were outside the room in different areas of the home and heard the discharge or heard what they believed to be a gunshot, and entered the room and found the child inside with the injury."
He was taken to Memorial Hermann Children's Hospital and immediately underwent surgery.
Officers say at this time they are not sure where the gun is normally kept inside the apartment. But the father acknowledged the gun was his and he kept it in the home for security purposes.
The boy's mother went with him to the hospital. His father was taken into custody. Officers say charges could eventually be filed against either one or both of the parents because the child had access to the gun.
Police say there has been a language barrier at the scene, so they will be conducting more interviews to find out exactly what happened.
No charges have been filed at this time, as the case is being presented to the District Attorney's Office for review.