Ranjit Bachchan was associated with the Samajwadi Party before he formed a right-wing group.
The police on Thursday arrested a right-wing Hindu outfit leader's wife, her lover and two others for his murder, officials said.
Ranjit Bachchan, 40, who had founded the little known Vishwa Hindu Mahasabha, was allegedly killed because he was "delaying" the proceedings to divorce his wife Smriti Srivastava, who wanted to marry her lover Deepender.
Their driver Sanjeet Gautam has been held in Lucknow for the murder. The man who actually pulled the trigger was arrested in Mumbai by an Uttar Pradesh police team.
Smriti Srivastava told police that Ranjit Bachchan was stalling their divorce proceedings by not appearing in court, Lucknow Police Commissioner Sujit Pandey said.
Ranjit Bachchan was shot dead on February 2 while he was out on a morning walk, barely a couple of kilometres away from the high-security Uttar Pradesh legislature complex in the heart of the state capital. His cousin Aditya Srivastava was also injured in the attack.
"Those involved in the murder are Smriti Srivastava (second wife), her friend Deependra, Sanjeet Gautam (driver) and Jeetendra (the shooter). Deependra was the person who convinced everyone to commit the crime. Smriti was part of the whole conspiracy. We arrested Sanjeet Gautam from Lucknow, Deependra has been arrested from UP-MP border and Smriti too has been arrested from Vikasnagar (in Lucknow)," Mr Pandey told reporters.
Giving details of the incident, Mr Pandey said, "Three people (Deependra, Sanjeet and Jeetendra) moved from Rae Bareli at 2.30 am (on the intervening night of February 1 and 2). On February 2, Deependra was dropped at Hazratganj crossing, the shooter was dropped near Capitol cinema hall crossing. At 5.40 am, Ranjit Bachchan, Kalindi and Aditya moved out from their home for morning walk. The shooter followed them in Hazratganj, and attacked them outside Globe Park."
Police said one vehicle was used for recce and another for escaping after committing the crime.
Based on specific inputs, the Uttar Pradesh's STF reached Mumbai on Wednesday night and arrested Jeetendra on Thursday with the help of their counterparts in Mumbai.
The Lucknow police commissioner said, "When we inquired about Ranjit Bachchan's family, we learned he had two wives. He was living with his first wife in Lucknow's OCR complex. A rape case had been registered against him."