Police were called to the incident at Invercauld Road in Aberdeen at 5.10 pm on Friday.
The child was taken to Aberdeen Royal Infirmary before being transferred to the Royal Hospital for Sick Children in Glasgow, where he is described as being in a stable but critical condition.
Inspector Simon Reid said: “Police and the Scottish Ambulance Service attended Invercauld Road after reports were received that a four-year-old boy had been injured after he fell from the window of a communal hallway within a residential block of flats.”
He added: “The boy’s family are aware and with him at hospital.
“Officers remain in contact with the family to provide support and have liaised with Aberdeen City Council regarding the incident.”