I'm trying to remember, where was the 4th when Roger was looking for them? Because I assume Big Mom wasn't in possession of hers at the time. It's entirely possible that Big Mom has the fishman one, considering that she "claimed" the island after Whitebeard died.
Edit: Ah, so, Big Mom's poneglyph is actually hinted at from Jinbe's cover story adventure that took place before he alliied with Big Mom. Which means the anime will probably never show this story.