Four Jupiters and out in the Oort cloud. It's possible that WISE could detect such a planet if it exists, according to Mike Brown in this Universe Today article.
My first response to seeing the tweet for this article was to say, Bah!, Humbug!, and ignore it. However, this theoretical outer body has some basis in patterns of comet orbits.
Actually, the most fascinating part of the article is the diagram. Look how evenly spaced the planets appear when put on a log scale! (Well, yes, with Uranus omitted).
Image originally from NASA/JPL.