Well, for a country that's never seen large scale air to air combat except maybe very limitedly during the Korean war, I suspect that 4+2 set up is plenty in the minds of Chinese military doctrine.actually happy you brought up this photo. I agree that missile almost touch the bottom which makes it unlikely to make room for additional missiles by shifting to the fore or to the back.
However, first note that there is still plenty of room on the side and also in between the missiles. The still available room is actually quite specious, and like mentioned before, especially if you compare to the cramped up bay from F22.
And second, which is even more interesting, note the angle in the back of the weapons bay, which is sloped down and backwards, meaning that there is still additional space both in the vertical direction as well as to the back when the bay is fully closed. And that room could be sufficient to allow you to arrange additional missiles in a stacked fashion: one additional missile in each half of the bay, hanging slightly down from the middle and to the back compared to the current ones (also note that we have photos going around almost since the very beginning showing potential additional attachment points in the bay). But like I said, unfortunately the angle from where the photos is taken does not permit precision measurement of the vertical space, so it is really hard to be conclusive. But you got to admit there is something there isn't it?
If there's any space left, I suspect the priority would be given to the future increase in missile size for longer range AWACS killers.