  Early time-series photometry from NASA's Kepler spacecraft has revealed a
planet transiting the star we term Kepler-4, at RA = 19h02m27.68s, Dec =
+50:08:08.7. The planet has an orbital period of 3.213 days and shows transits
with a relative depth of 0.87 x 10^{-3} and a duration of about 3.95 hours.
Radial velocity measurements from the Keck HIRES spectrograph show a reflex
Doppler signal of 9.3 (+1.1 -1.9) m/s, consistent with a low-eccentricity orbit
with the phase expected from the transits. Various tests show no evidence for
any companion star near enough to affect the light curve or the radial
velocities for this system. From a transit-based estimate of the host star's
mean density, combined with analysis of high-resolution spectra, we infer that
the host star is near turnoff from the main sequence, with estimated mass and
radius of 1.223 (+0.053 -0.091) solar masses and 1.487 (+0.071 -0.084) solar
radii. We estimate the planet mass and radius to be 24.5 +/- 3.8 Earth masses
and 3.99 +/- 0.21 Earth radii. The planet's density is near 1.9 g/cm^3; it is
thus slightly denser and more massive than Neptune, but about the same size.
