Complex sentence: Chang'e 4 will be China's second lunar lander and rover, and was built as a backup to Chang'e 3, as Chang'e 2 was to Chang'e 1.
Split into simpler sentences: Chang'e 4 will be China's second lunar lander and rover. It was built as a backup to Chang'e 3, as Chang'e 2 was to Chang'e 1.