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+/-0.08, Teff=5650+/-75K, and logg=4.4+/-0.1 for TrES-3, and [Fe/H]=+0.14+/-0.09, Teff=6200+/-75K, and logg=4.0+/-0.1 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. With these revised stellar parameters, we obtain improved values for the planetary masses and radii. We find M_p_=1.910^+0.075^-0.080_M_Jup, R_p_=1.336^+0.031^-0.036_R_Jup for TrES-3, and M_p_=0.925+/-0.082M_Jup_, R_p_=1.783^+0.093^-0.086_R_Jup for TrES-4. We confirm TrES-4 as the planet with the largest radius among the currently known transiting hot Jupiters.