Asif Ali Zardari, co-chairman of the party expected to lead the new government, condemned the bombing in the eastern city of Lahore as "inhuman, barbaric and most despicable."
Pakistanis must unite against "conspirators and extremists," said Zardari, the widower of opposition leader Benazir Bhutto, who was assassinated in a gun and suicide bomb attack in December.
Officials provided differing accounts of what happened.
City police chief Malik Iqbal said two bombers rode up to the gate of the parking lot of the Pakistan Navy War College on a motorcycle.
The passenger dismounted and destroyed the gate, allowing his accomplice to ride inside and unleash a much more powerful blast, Iqbal said, discarding earlier accounts that the bombers had been riding in a delivery truck.
"The first blast was low intensity and was meant to crash the gate. The other blast was high-intensity and was aimed at causing big losses," Iqbal told reporters at the scene.
He said the four dead and 14 wounded were all employees of the college. None were senior military officials, he said.
However, an Interior Ministry spokesman said a car bomber had rammed a navy bus on its way into the compound and other explosions were caused by the fuel tanks of parked vehicles.
"There would be no let up in the campaign to root out extremism and terrorism from the country, which poses the gravest threat to our national security," spokesman Javed Iqbal Cheema said at a news conference in the capital.
"The whole nation is full of contempt for these anti-state, anti-Islam perpetrators of violence," he said.
The police chief said investigators were still gathering body parts and debris from the scene and that it was too early to speculate about who was responsible.
Earlier, television footage showed black smoke billowing from inside the college compound and several injured people with bloodstained clothes walking out. Two wrecked cars and a half-dozen damaged buses were visible behind the mangled metal gates.
Muhammad Safdar, a 23-year-old chauffeur, said he had dropped off an officer at the college and was sitting in the canteen when he heard the first blast and rushed outside.
"There was smoke and vehicles on fire ... several people were lying on the ground injured and crying for help, but I too was injured as something hit me in the neck," Safdar told reporters.
Security personnel quickly sealed off the area, a high-security neighborhood that includes the state governor's residence, a luxury hotel and several government offices.
The college trains senior naval officials from Pakistan and other countries.