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, T eff = 5650 ± 75 K, and log g = 4.4 ± 0.1 for TrES-3, and [Fe/H] = +0.14 ± 0.09, T eff = 6200 ± 75 K, and log g = 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. 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 sstarf (related to the stellar density) in conjunction with our new temperatures and metallicities. The masses and radii we derive are M sstarf = 0.928+0.028 -0.048 M sun, R sstarf = 0.829+0.015 -0.022 R sun, and M sstarf = 1.404+0.066 -0.134 M sun, R sstarf = 1.846+0.096 -0.087 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 Mp = 1.910+0.075 -0.080 M Jup, Rp = 1.336+0.031 -0.036 R Jup for TrES-3, and Mp = 0.925 ± 0.082 M Jup, Rp = 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.