The infected patient in Dorbod Banner, who has been isolated, is being treated in a local hospital and is in stable condition, according to a statement from the health commission.
Experts have pointed out that the plague is preventable and controllable. Local authorities are implementing rodent and flea control, and publicity work. After being misdiagnosed at the Inner Mongolia hospital, the couple were diagnosed with pneumonic plague and treated at a specialist hospital in the Chinese capital. Earlier this month three people from Inner Mongolia were hospitalized for plague in two separate cases that were linked to eating wild animals.
Among different types of the plague, the Bubonic plague is the most common one; despite being less risky than other strains, it's infamous for having caused the so-called Black Death during medieval times.
Following the discoveries, the authorities urged citizens to maintain good personal hygiene and stay away from wild animals as they conducted a flea and rat eradication in the area where the fourth patient was found.
Ulanqab officials didn't disclose the specifics of how the latest patient caught the plague, but said the patient had been "active" in a plague-affected location prior to falling ill.
The Yersina pestis bacteria that causes plague is endemic to Inner Mongolia.
Though the highly-contagious plague is rare in China, several cases have proven deadly in the past few years.
In 2014, a man died of the plague in northwestern Gansu province and sparked the quarantine of 151 people.