Apparently China was part of the development of the S-400. They might use some of the technology to upgrade their HQ-9's..China know Russian are unreliable weapon supplier. And buying Russian weapon as a stop gap only.
Since the last delivery of S-300PMU2, which important complete weapon system has China bought from Russia? Besides some AL-31F engines, nothing new is bought.
If there is one thing worthy to buy. If will be S-400 triumph. But seems so far, China has not make any move for it.
Russian naval weapoexports for China I think are now practically non existant. But transport aircraft like the Il-76 and some helicopters (as well as technology assistance in some areas) still have a place in China.