Solomon Jordan Smith, who was known as Jordan by relatives, was pronounced dead following the shooting at a house on the north side last Friday.
Authorities said based on evidence and his injuries, they believe the child shot himself.
She has been released and ordered to meet with prosecutors in the Milwaukee County District Attorney’s Office next week.
The shooting occurred in the 2100 block of North 15th Street when a gun was fired inside in the home.
Jordan was struck by a bullet and pronounced dead at the scene, and a gun was found inside, authorities said.
Neighbors and loved ones left balloons and candles outside the home on Friday in a makeshift memorial for Jordan, The Milwaukee journal Sentinel reported.
The family also held an emotional vigil on Sunday for the four-year-old where pictures of him in happier times were displayed outside the home.
‘He was a beautiful child. Everybody is devastated at this time,’ Rufus Woods, Jordan’s step great-grandfather, told Fox 6
‘Oh my God. Pray for us. Mistakes happen and that little boy loved his mother,’ she said.
Jordan is the third child younger than 13 to be fatally shot in Milwaukee this year, The Milwaukee Journal Sentinel reported.