...Four? Really? Or did you mean three?
I've looked everywhere and am positive the dev isn't kidding... I'm pretty sure the sequence of keys has nothing to do with obtaining a fourth one at all, or that it even exists. But the ingame message about not looking for any more after three was so oddly insistent that I'm half-convinced it's a bit, there just to throw you off. Then there's the "moderate" rating on the formula at the end of the game after consuming 3 milks, which leads me to believe you can do better somehow.
...Also becoming immobile would be a pretty funny end... and a nice secret capstone for a breast expansion focused chapter.