The vehicle was carrying more than 30 Chinese engineers to the Dasu Dam project site in Upper Kohistan when the attack took place on Wednesday morning.
Inspector General Moazzam Jah Ansari, a police official in Khyber-Paktunkhwa province where the incident occurred, told Reuters news agency that those killed included two soldiers and two residents. Several people were injured.
The dead and injured were taken to the Dasu regional health center.
Another senior government official told the news agency that the bus fell into a deep ravine after the explosion and authorities launched a rescue operation.
High Kohistan Deputy Commissioner Arif Khan Yousafzai said Dawn that the cause of the incident has not yet been determined and the site has been cordoned off while the investigation continues.
A police official said it was difficult to get information because mobile networks do not work in the area.
Condemning the explosion, China called on Pakistan to conduct a full investigation into the incident. Its foreign ministry said Pakistan must stop the attackers and protect the safety of the Chinese people and institutions there, according to China daily.
Earlier in June, at least 17 people were reportedly killed after a passenger van plunged into a river in Upper Kohistan district. The incident occurred when the driver lost control of the vehicle, officials said at the time.