From looking at the pictures of the opened weapons bay it appears that the bay is not uniform in its depth. It appears deeper towards the centre compare to either ends. Maybe they could stake 6 AAM via some overlay arrangement or it could be for other larger circumference munitions etc.
I'm not sure if the bay's depth is actually not uniform if it is just the appearance based off the S duct overhead making the weapons bay look deeper at certain parts.
But even if the depth was not uniform, I doubt that they would overlay AAMs, as such a choice would present a risk that failing to launch a given AAM means every other AAM in that half of the weapons bay is blocked up behind it.... and even if they did go with that decision I don't think there's enough continuous vertical height to attempt two layers of AAMs inside the weapons bay anyway.
The key thing to look at is whether there has been any change in the LRIP birds' (as well as 201X prototypes by extension) width compared to the weapons bay of 2002 (and 2001 by extension).
We should hope that we end up getting pictures of the J-20's weapons bay at similar angles to 2002's weapons bays from a few years ago to compare.
And if it turns our the weapons bay is only able to carry four PL-15s internally, I would not be surprised if a smaller diameter/wingspan MRAAM emerges in the coming years to better optimize J-20's carriage of such weapons, because I doubt the Air Force would be content with less internal MRAAMs than other stealth fighters of its class.