The attacks come a day after leaked documents showed US officials believe Pakistan's military intelligence agency to be a terrorist group.
A navy spokesperson said the four dead were all navy officials including a female doctor, a sub-lieutentant, a sailor and a civilian employee. The first bomb was planted on a motorbike parked in the upmarket Defence Housing Scheme and the second was hidden in rubbish in the impoverished neighbourhood of Baldia.
Following the twin blasts, provincial government officials blamed extremist Islamist organisations linked to the Taliban and Al-Qaeda such as Lashkar-i-Jhangvi which was involved in the murder of US journalist Daniel Pearl in 2002.
And in the neighbouring province of Baluchistan, armed men attacked and set fire to a passenger bus on Monday killing at least 13 people in an incident unrelated to the explosions in Karachi.