XO-4 is a Sun-like F-type star (about 1.10 solar masses, 6,397 K, 1.45 solar radii), 273 light-years from Earth. XO-4 b is a 1.42-Jupiter-mass companion, with an orbital period of 4.1 days, discovered in 2010 via transit. The inner world is a hot Jupiter — a gas giant hugging its star at a fraction of Mercury's distance — a class that forced astronomers to rethink how giant planets form and migrate inward. The planet's large radius (14.0 Earth radii) indicates a bloated atmosphere heated by intense stellar radiation.