Banwarli Lal, his sons and his nephew were found dead in their rented accommodation in the Shivaji park area in Alwar on Tuesday morning, they said.
The investigation has led to the arrest of Mr Lal's wife Santosh, in her mid-30s, and her paramour Hanuman Jat (25), and two others Kapil and Deepak, the police said.
They were killed with sharp-edged weapons when they were fast asleep, they said.
On Monday night, she mixed some substance in their food and served it to them, he said.
"The same night Hanuman, Deepak and Kapil entered the house after Santosh opened the door, and killed them in their sleep. The woman went to another room and slept," the officer said.
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