The Centre for Health Protection is investigating Legionnaires' disease cases involving four men aged 48 to 85.
Three are from Hong Kong and one is a visitor from Dongguan. All of them have underlying illness.
Investigations are ongoing to identify potential sources of infection.
Fifty-one Legionnaires' disease cases have been reported so far this year. There were 66 cases in 2015 and 41 in 2014.
