The incident occurred around 1 p.m. when the labourers were working on the 16th floor of the 24-storied building coming up at Soniwadi, near the Kalpana Chawla Chowk, in Borivali suburb of north-west Mumbai.
The BMC said that the unidentified victims were rushed to the Shatabdi Hospital in Kandivali west where three were declared dead and one is stated to be in a critical condition.
The Mumbai Police and fire brigade teams rushed to the spot and the causes leading to the scaffolding crash are being investigated, whether they were equipped with safety gear or not, plus ascertaining the identity of all the victims.