At least one of the victims was shot with a revolver recovered from the scene of the crime in Shiravali village, while the heads of three others were smashed with a stone that was also found there.
The victims have been identified as Nitin Joshi, Balram Chaupanne, Pritam Gharat and Ramdas Patil, all hailing from Panvel, police said.
The farmhouse was owned by Datta Patil, a local agriculture group activist, whose nephew - Ramdas Patil - was among those the four killed, police said.
The four were reported to be missing since Sunday, police said.
Panvel, an important junction on the Mumbai-Pune and Mumbai-Goa routes, is around 30 km southwest of Mumbai.