I think I responded to your reddit post already, but I'll reiterate what I said there, here:
I would be very surprised if the California sea lion is not the last animal, due to it's abundance in captivity and popularity amongst zoo guests. The Arctic pack did not have an exhibit animal, and I doubt they will add mini aquarium tanks just yet. I would love a turtle with a brand new exhibit biome to suit it, but I don't expect it. I think four animals is, unfortunately, our reality.
That being said, if we are fortunate enough to get 5 or 6 animals, I would guess that it would be the Asian Short-Clawed otter and Cape Penguin (aka South African penguin) due to their abundance in captivity and popularity. They would also be able to use the same rigs and animations as the Giant otter and King penguin, which I am personally considering as confirmed animals due to all the circumstantial evidence. Of course, that also operates under the assumption that those three animals will all be in the same pack, probably called the "Aquatic pack" or something, but who knows, it could just be the Gray seal and King penguin, plus two unknown animals in a "Marine pack". We'll have to wait and see.
Side note: If those three animals are in fact all going to be in the same pack, it will be interesting to see how Frontier decides to reveal the remaining unknown animals. Will they do one animal a day, like the South America and Arctic packs? Or will they reveal them all at once, like the Australian pack?