To his parents' horror, Kenzie Joe Jubb was missing for 11 minutes before teachers realised the wrong child had been sent with a stranger to 'see his father'.
Now an investigation has been launched after Kenzie's dad hit the roof when he found out what had happened.
The little boy was removed from his nursery classroom at Francis Askew Primary School in Hull, East Yorkshire, and sent with a stranger to 'see his father' to social services offices - along with a two-year-old girl they mistakenly thought was his sister.
Kenzie's dad, Alistair Jubb, told the local paper: "We are furious. The school CCTV shows him being away for 11 minutes, but 11 minutes is a long time when your son is with someone he doesn't know.
"How did they get the wrong child? We want to know exactly how this has happened.
"I am absolutely furious with the school. The head has assured me this will never happen again, but I wouldn't have ever expected it to happen in the first place.
"Somebody has messed up somewhere along the line and we want to know who it was and how it happened."
The school contacted the city council's Safeguarding Children Board when they realised the error and an investigation is being carried out.
Alistair said: "The school called his mum and she was distraught. When she called me, all sorts went through my head.
"She said Kenzie had been taken out of school and the first thing that went through my head was that it was someone with a fake identification.
"It was horrific. His mum said he was back in school and those feelings of horror turned to anger.
"Kenzie said he was put in the car and the social worker told him he would be going to a house where there was a dog and he would be seeing his dad.
He is only four. He was confused. He was taken by someone who he has never seen before in his life.
Hull City Council confirmed a joint investigation will take place between themselves and the school. John Readman, director of children's services, said: "This incident should not have occurred. Both the school and the local authority have apologised personally to the families involved for the distress this must have caused.
"Importantly, the child was at all times supervised by an experienced member of staff. All staff that work directly with children are subject to enhanced CRB checks and required to carry identification, as in this case.
"We now need to understand exactly what happened in this instance and ensure that correct procedures are followed at all times. "The school and the local authority are jointly investigating the incident."