To perform successful research on far side of the moon, Change-4 will utilize data relay satellite which will be launched by China National Space Administration 6 months before Change-4.
After Chinese announcement about planned launch of next lunar probe, Change-4, CNSA decided to use during this mission additional data relay satellite. Change-4 is lunar probe consisting lander and rover; it was designed and manufactured as potential backup for Change-3 launched on 2013. Change-4 is scheduled for 2018 and will be send to perform research over dark side of the moon, study geological of far side of the Moon and test science equipment which will be installed on Change-5. Probe landing zone was not yet announced but CNSA is taking under consideration Aitken Basin. It is largest known impact crater in solar system with 2500 km of diameter and 13 km deep.
Landing in the far side of the Moon brings problems with communication with Earth. From this part of the Moon there is no visible contact with Earth and radio waves could not be received during transmission to or from Earth. To solve this problem, CNSA will send data relay satellite which will receive data from Moon and send it to Earth serving as a retransmission point. This satellite will be placed in the distance of 80000 km from the Moon, on the Lagrange point L2 of the Earth-moon system. There is no details about technology which will be utilized by data relay satellite or its construction.