The victims were illegal immigrants and were being taken to Iran for travel to Europe through Turkey, The Express Tribune reported.
"Four bodies were brought to the District Headquarters Hospital Gwadar by local fishermen," District Police Officer (DPO) Gwadar Pervez Umrani was quoted as saying, adding that the incident is being investigated.
The DPO said there could be more people who drowned, but their bodies are yet to be pulled out.
One of the victims was identified as Mohammed Basheer, a resident of Mansehra in north-western province of Khyber-Pakhtunkhwa.
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According to a fisherman who took part in the rescue operation, the victims were illegal immigrants.
The speedboat had a capacity of holding eight to 10 people, and it could be possible that there were more than 12 people onboard.
However, the reason of the incident could not be ascertained immediately.
According to local officials, dozens of illegal immigrants try to cross the border illegally via speedboats.