“The man was not taken into custody as a suspect and was questioned as an ordinary citizen,” Colonel Noxolo Kweza said on Tuesday.
No arrests were made and police were still searching for the hijackers.
On Monday, Kweza said police hoped the man questioned would be able to provide information which could help the investigation.
She denied a report that a man linked to the hijacking had handed himself over.
The boy, Taegrin Morris, died after being dragged for a long distance behind a hijacked car on Saturday night.
He was travelling in a VW Golf with his mother, father, and sister in Reiger Park when the family was approached by three men.
“They ordered him [the father], his wife and daughter to get out of the vehicle,” Lt-Col Khensane Magoai said at the weekend.
“The four-year-old was left in his safety belt. They [the hijackers] got in the car and drove away.”
Magoai said it was unclear how it happened but the boy was then dragged along, hanging outside the car door by his safety belt.
“He was dragged along until Boksburg. The hijackers then abandoned the vehicle and the boy was dead.”
Police had offered a reward of R50,000 for information on the hijacking.