An official said that the speedboat was on its way from Balochistan province's Gwadar district to the Iranian border when it capsized, Dawn online reported.
The four bodies were retrieved from the Arabian Sea and a search operation initiated by the locals is underway to locate the seven other persons or their bodies, the official said.
The four bodies are yet to be identified.
"The boat was overloaded and all of a sudden it somehow sank", the official said.
He said the victims were illegal immigrants from Pakistan trying to enter neighbouring Iran.
Thousands of Pakistanis trying to enter Iran illegally are caught every year by Iranian border guards.
Iranian authorities have often been deporting Pakistanis entering their country illegally.
Unemployed and poverty-stricken Pakistanis in search of jobs sneak into Iran via unfrequented routes, found within the 900 km border.
They travel through the deserts of Iran and Turkey on their way to Greece.
Very few manage to reach that country, with the majority getting caught either in Iran or Turkey.