The toll in the blast at a bulk drug unit of Hetero Drugs Limited at Nakkapalli near here rose to four with one more injured patient succumbing to the burns at a corporate hospital here on Thursday.

Papa Rao, one of the workers injured in the blast, died following multi-organ failure. The blast occurred on January 5 when heavy pressure built up in a reactor.

Joint Collector Praveen Kumar is inquiring into the circumstances that led to the blast.