A female suspect was apprehended at a shopping mall close to Quarry Bay Park where the incident occurred, the reports said.
Citing police, the website of the popular Apple Daily said the four victims were brothers and sisters in their sixties and seventies and that the assailant was a relative. It cited police as saying the shooting was motivated by a financial dispute, AP reported.
It said two suffered head wounds while the other two were shot in the arm. There was no immediate word on their condition.
Firearms are tightly controlled in Hong Kong and such shootings are highly unusual, usually restricted to armed robbers and members of organized crime groups known as triads.
The Asian financial hub is considered generally safe, although thieves have sometimes attacked jewelry stores using hammers.
According to police statistics, 10 murders were recorded between January and April this year, up by two from the same period last year. It wasn't clear whether firearms were used in any of those killings.