  We report the discovery of TrES-4, a hot Jupiter that transits the star GSC
02620-00648 every 3.55 days. From high-resolution spectroscopy of the star we
estimate a stellar effective temperature of Teff = 6100 +/- 150 K, and from
high-precision z and B photometry of the transit we constrain the ratio of the
semi-major axis and the stellar radius to be a/R = 6.03 +/- 0.13. We compare
these values to model stellar isochrones to constrain the stellar mass to be M*
= 1.22 +/- 0.17 Msun. Based on this estimate and the photometric time series,
we constrain the stellar radius to be R* = 1.738 +/- 0.092 Rsun and the planet
radius to be Rp = 1.674 +/- 0.094 RJup. We model our radial-velocity data
assuming a circular orbit and find a planetary mass of 0.84 +/- 0.10 MJup. Our
radial-velocity observations rule out line-bisector variations that would
indicate a specious detection resulting from a blend of an eclipsing binary
system. TrES-4 has the largest radius and lowest density of any of the known
transiting planets. It presents a challenge to current models of the physical
structure of hot Jupiters, and indicates that the diversity of physical
properties amongst the members of this class of exoplanets has yet to be fully
explored.
