In the case of the killing of police inspector Subodh Kumar Singh and a 20-year-old bystander Sumit Kumar in mob violence in Bulandshahr, police on Tuesday arrested four people and were hunting for a Bajrang Dal activist Yogesh Raj, even as Chief Minister Yogi Adityanath has ordered a thorough probe and issued directions for strict action against those involved in the alleged cow slaughter, said to be the trigger for the brutal incident.