There are two kinds of post-doctoral fellowships available in China. The more traditional Chinese post-doctoral fellowship offers you a government-recognised certificate at the end of training. The more "Western" style does not. There are additional differences having to do with employment, benefits and such that are too individual to list here. Regardless, both are two year contractual commitments.
In general, the more traditional route is very strict in its requirements. You must meet age and graduation requirements and register through a common government portal. The strictness is due, in part, to closely aligned visa requirements. Thus, even if the institution to which you are applying is willing to allow you to attend four years after your PhD, the visa bureau is likely to decline this application.
The more "Western" style of fellowship is much more fluid because it is, in part, more structured like an employment contract and less a training scheme. In addition, there is no need for the academic institute to issue you a government-recognised training certificate. Under this route, there is more flexibility in entry requirements. However, this route is much, much less popular in China. It may not exist in some fields of research.