SSP Chaudhary Aslam told reporters that they conducted a raid near Ghani chowk area in the city in which the four militants were arrested.
"They belong to a particular group of the Taliban which specialises in bomb making. One of the arrested militant, Asim has said they were planning to carry out a bomb attack against former President Pervez Musharraf who is in Islamabad," he said.
"All the four have received training in the tribal areas and came to Karachi to carry out terrorist activities in the city," he said.
In another operation, the police arrested four notorious target killers from the port city.
Pakistan has witnessed a spike in violence ahead of the May 11 general election, which will mark the country's first democratic transition of power.