The four persons have been arrested by the Delhi Crime Branch which is hunting for more of their associates, a senior police official said.
38-year-old Singh was attacked by over six armed men last Sunday evening when he was attending a religious programme at Abhinandan Vatika in Najafgarh. At least 15-20 rounds of bullets were fired at him.
The attackers had reached the spot in a SUV which was recovered from Kharkunda in Haryana on Wednesday.
From day one, the needle of suspicion has been on two gangs who have had reasons to eliminate Singh, one is the gang of Vikas Lagarpuria and the other is of Udaiveer alias Kaale and the investigation so far indicates that the two came together for the "operation".
More From This Section
Kaale's father and brother were allegedly bumped off by Singh and his brother's gang.
It is believed that Udaiveer, who was recently released from jail hired UP-based hitmen to kill Singh.
Kaale is also believed to have made a failed attempt to eliminate Singh in 2012.
In 2012, Singh, who was then a sitting MLA escaped narrowly by hiding in the bathroom when eight gunmen shot at him in his office on a June morning.
He had received three of the 54 shots fired at him but he survived to tell the tale after spending nearly two months in a hospital and losing a kidney.