Friends and relatives of the latest victims urged the government to intervene to explain how the four ended up in a river, around 40 kilometers the capital.
Multinational food company, Del Monte, said its local unit was cooperating with the authorities and alleged there was rampant organized crime around pineapple theft in the region.
"Our security footage from when the men attempted to steal pineapple shows no foul play on Del Monte's part and instead shows the thieves running away towards the river... as they tried to run away from security guards," it said.
Earlier this year a joint investigation by Britain's Bureau of Investigative Journalism and the Guardian newspaper found evidence they said showed security guards working for Del Monte had killed and brutally assaulting villagers suspected of trespassing on the Kenyan pineapple farm.
Del Monte said at the time it had initiated investigations, supported by an independent review by a specialist human rights consultancy.
"Kenyan police have launched investigations into the killings of people inside the Del Monte pineapple farm," said David Kainga Mathiu, Muranga county police commander.
After five days of searching, two bodies were found in the river on Sunday, and another two on Monday, independent Nairobi-based broadcaster Citizen TV reported.