The body of Ash Wimbey, from Indiana, Illinois, was discovered on Thursday morning at Tubby's Wine and Spirits store.
He is said to have found the loaded handgun behind the store counter and shot himself in the head.
He was rushed to hospital but was pronounced dead on arrival.
Officials are still trying to figure out whom the gun belongs to and how the four-year-old was able to access it.
He is the fourth child in the area to die in a gun-related accident.
An 11-year-old is accused of fatally shooting his six-year-old brother inside the family home in Martinsville on June 30.
Another incident took place in Connersville on July 14 when a two-year-old died after a gun went off while he was playing with his five-year-old brother.
On July 18, an 11-year-old boy in Clinton, Indiana, suffered a gunshot wound to the head while he was playing with friends.
The boy was last listed in critical condition in an Indianpolis hospital.
Beecher police said in a statement: 'Our thoughts and prayers go out to the family and to anyone else who has been affected by this tragic incident.' Read More