Kim Sung-hwan told the family of one of the activists he plans to press Beijing to release the men as soon as possible, South Korea's Yonhap News Agency reported Thursday.
The four were arrested by Chinese police March 29 on charges of endangering China's national security. They are being held in the border city of Dandong across the Yalu river from North Korea.
Their detention drew media attention this week after the Network for North Korean Democracy and Human Rights issued a news release demanding China free the detainees.
One of the men being held, Kim Young-hwan, 49, is a senior researcher for the organization.