  We describe the discovery of HAT-P-4b, a low-density extrasolar planet
transiting BD+36 2593, a V = 11.2 mag slightly evolved metal-rich late F star.
The planet's orbital period is 3.056536+/-0.000057 d with a mid-transit epoch
of 2,454,245.8154 +/- 0.0003 (HJD). Based on high-precision photometric and
spectroscopic data, and by using transit light curve modeling, spectrum
analysis and evolutionary models, we derive the following planet parameters:
Mp= 0.68 +/- 0.04 MJ, Rp= 1.27 +/- 0.05 RJ, rho = 0.41 +/- 0.06 g cm-3 and a =
0.0446 +/- 0.0012 AU. Because of its relatively large radius, together with its
assumed high metallicity of that of its parent star, this planet adds to the
theoretical challenges to explain inflated extrasolar planets.
