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] $= -0.19pm 0.08$, $T_mathrm{eff} = 5650pm 75$ K, and $log g = 4.4pm 0.1$ for TrES-3, and [Fe/H] $= +0.14pm 0.09$, $T_mathrm{eff} = 6200pm 75$ K, and $log g = 4.0pm0.1$ 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.
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