  We report new spectroscopic and photometric observations of the parent stars
of the recently discovered transiting planets TrES-3 and TrES-4. A detailed
abundance analysis based on high-resolution spectra yields [Fe/H] $= -0.19\pm
0.08$, $T_\mathrm{eff} = 5650\pm 75$ K, and $\log g = 4.4\pm 0.1$ for TrES-3,
and [Fe/H] $= +0.14\pm 0.09$, $T_\mathrm{eff} = 6200\pm 75$ K, and $\log g =
4.0\pm0.1$ for TrES-4. The accuracy of the effective temperatures is supported
by a number of independent consistency checks. The spectroscopic orbital
solution for TrES-3 is improved with our new radial-velocity measurements of
that system, as are the light-curve parameters for both systems based on newly
acquired photometry for TrES-3 and a reanalysis of existing photometry for
TrES-4. We have redetermined the stellar parameters taking advantage of the
strong constraint provided by the light curves in the form of the normalized
separation $a/R_\star$ (related to the stellar density) in conjunction with our
new temperatures and metallicities. The masses and radii we derive are
$M_\star=0.928_{-0.048}^{+0.028} M_{\sun}$,$R_\star = 0.829_{-0.022}^{+0.015}
R_{\sun}$, and $M_\star = 1.404_{-0.134}^{+0.066} M_{\sun}$,
$R_\star=1.846_{-0.087}^{+0.096} R_{\sun}$ for TrES-3 and TrES-4, respectively.
With these revised stellar parameters we obtain improved values for the
planetary masses and radii. We find $M_p = 1.910_{-0.080}^{+0.075}
M_\mathrm{Jup}$, $R_p=1.336_{-0.036}^{+0.031} R_\mathrm{Jup}$ for TrES-3, and
$M_p=0.925 \pm 0.082 M_\mathrm{Jup}$, $R_p=1.783_{-0.086}^{+0.093}
R_\mathrm{Jup}$ for TrES-4. We confirm TrES-4 as the planet with the largest
radius among the currently known transiting hot Jupiters.
