With 4 prototypes, is there any chance of a multi aircraft flight? A 4 plane flight would be something to behold.
At least one of the airframes (probably the first) was seen on satellite out in Western china as part of its test program; unless there's a reason for it to return to Chengdu or for all four (if there are four) prototypes to fly together, I can't see this happening.
Besides, a four ship flight would just be a flex rather than useful for test purposes.