I think they just haven't gotten around to listing the fourth pony yet. They have until September so why rush.
BTW for some reason the pictures on the listings are from customs on DeviantArt?? I don't know if BF thought they were real Hasbro prototypes (they look REALLY professional) or if an intern posted the wrong pics or what.
So don't be surprised if the real ponies are in different poses, lol. I imagine the pics will be updated later.
But yeah! This is confirmation that this set is happening AND will be for sale on Amazon!!