The explosion took place on Monday night in the city's Mini Market area shortly after the police van arrived at the mosque to provide security to the people offering prayers.
The Tehreek-e Taliban Pakistan (TTP) claimed responsibility for the blast, just days after its splinter group Hizbul Ahrar claimed the attack on the police guarding Pakistan's oldest Sufi shrine in Lahore.
"The police vehicle carrying personnel for mosque security was targeted in the blast in which our four personnel of Rapid Response Group (RRG) lost their lives, while the condition of another was stated to be serious," Quetta Deputy Inspector General Razzaq Cheema told the Dawn newspaper.
Many vehicles parked in the area were damaged and windowpanes of nearby buildings were shattered due to the impact of the powerful bombing though the mosque remained safe, the report said.
Security forces cordoned off the area and launched a search operation to trace the elements involved in the blast.
This was the second major terror attack in Balochistan within the last three days, as on Saturday a luxury hotel in Gwadar was targeted by gunmen. Five people were killed in the incident.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)