First milk is in the Hilly House's kitchen, second is hidden behind some crates on the left in the Apple shooter mini game, and third is in the Wellhouse. You need to finish the mini games to collect the latter two.
Second key goes to a chest in PawPaw's room, also in the Hilly House
Viewing post in Any got a list of milk locations?
...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.
You know, I was actually worried that my insistence there would make people think it was a bit lol But no it's genuine, there are only three bottles to be found.
And there is actually an immobile ending, but you need to find all the milk bottles for it. You can either return to the Base with all three, or you can drink one and mix it with the other two regular milks. I don't remember my reasoning for the weird formula, but there's a chance I'll revisit the number of milks and revamp the general formula for it.