A couple of other persons were feared trapped in the debris, Ulhasnagar tehsildar (revenue officer) Komal Thakur said. Rescue and relief operations were underway, the revenue officer said.
A slab of the third storey of the building located in Ulhasnagar camp 5 collapsed around 11.30 am, adding the structure having 30 flats was illegal and had already been served a notice.
Five families were still staying in the building, the official said.
As a portion of the building collapsed, local firemen, disaster management team, police, revenue and civic officials were pressed into service and started the rescue and relief operations, the official said.
The four deceased included two women. They were identified as Sagar Occhani (19), Priya Dhanvani (24), Renu Dholandas Dhanvani (54) and Dholandas Dhanvani (58), the official said.