Violence has not returned to the district known as the country's sugarcane-belt, but political divisions remain as Hindus typically vote for BJP and Muslims for the opposition
Tehrik-i Taliban Pakistan (TTP), the Pakistani Taliban, claimed responsibility for the blast in an emailed statement, just days after they claimed an attack on police guarding a major Sufi shrine in the eastern city of Lahore.
Security forces have been on high alert during the Muslim holy month of Ramadan, with extra security set up at major sites around the country.
Ziaullah Langove, home affairs minister in the province of Balochistan, said Monday's bomb targeted police guarding the mosque, where people were performing evening prayers.
Quetta police chief Abdul Razzaq Cheema said the four dead were all police and there were two police among the wounded.