The elder son was confirmed to have had the virus but has since recovered. He is one of five people who have been discharged from hospital after treatment, according to Xinhua.
So far, one boy is confirmed to have been an asymptomatic carrier of the virus, meaning he tested positive for the illness but didn't display any symptoms. The discovery of an asymptomatic carrier is worrying because it could make the spread of the infection more difficult to monitor, experts say.
The 4-year old was part of a sweep of people tested for the illness because they'd come into close contact with the first reported case in Beijing.
Authorities took throat swabs from a group of people connected to 24 poultry farmers in Naidong Village, Cuigezhuang County in Beijing's Chaoyang District, according to the Beijing Municipal Health Bureau.
The WHO reported that of more than 80,000 birds tested in China, less than 40 had tested positive.
Unlike previous outbreaks of other strains of bird flu, none of the infected birds showed signs of being sick.