Kanakaraj died after his two-wheeler hit a car at Athur in Salem district, the police said.
Another suspect and close friend of Kankaraj, Sayan was seriously injured while his wife and child died in a road accident involving their two-wheeler and a car in Kerala's Thrissur district today, they said.
Sayan has been admitted to a hospital in Coimbatore.
The estate figures in the disproportionate assets case involving jailed AIADMK general secretary V K Sasikala and Jayalalithaa, among others.
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A security guard posted at the bungalow at nearby Kodanad Estate was found dead on April 24, while some documents were suspected to have been stolen from there.
While 40-year-old Om Bahadur was found dead, another guard was found seriously injured.
Superintendent of Police Murali Ramba told reporters at Udhagamandalam that investigation has revealed involvement of 11 persons in the case, out of which four have been arrested today.
The four arrested have been identified has Santhosh Samy, Deepu, Sathishan and Uday Kumar, all from Thrissur in Kerala. Five others are still at large, he said.
The four were taken into custody yesterday for questioning. Their involvement in the crime has been proved through meticulous investigation and collection of scientific evidence, the officer said.
All of them had planned the dacoity based on Kanakaraj's information that a huge amount of cash was reportedly there in the bungalow, the official said, adding they used three cars to go to the site of crime.
Later, they broke open the doors of three rooms of the bunglow, including those used by Jayalalithaa and her confidante Sasikala. They could not find any cash but took away five watches and a crystal showpiece, the officer said.
The crystal showpiece has been recovered. The arrested accused said they threw the watches in a river in Kerala, he said, adding investigation is underway.
Judicial Magistrate S Senthil Kumar has held inquiries with Sayan, at the hospital regarding the accident, police said.