At this point of time, I really stopped being impressed with whatever Russia could sell to China. Time really does matters in these kind of sales, every year of delay means another year of R&D on that particular field for China.
By the time the S-400 does makes its appearance in China, chances are that China would already have a domestic equivalent that is at least as good, if not better than what they were offering in the first place. Even if there isn't the R&D done would have seriously lessen whatever chances there is of future purchases.
The Su-35 purchase was perhaps one of the most drawn out and painful sales pitch that Russia could ever do. And a one off sum of 2 billion seems hardly worth it in the long run. But there is a lot more to the deal that we are not privy to.
Russia really needs to pick up a book on how to be a better salesman..................