We report on the temperature-dependence of the infrared (ir) and Raman active modes of 4H-SiC. From ir reflectivity spectra collected at 7K we resolve, for the first time, the dispersion of the dielectric constant associated with the weak longitudinal / transverse splitting of the E-1 (FTA) frequency at 265 cm(-1) and the A(1) (FLA) one at 610 cm(-1). Next, we discuss the temperature dependence. We show that, in the temperature range 20 K-1000 K, similar to 80 to 95 % of the thermal shift comes from the effect of lattice dilatation. The remaining contribution comes from a weak three-phonons anharmonic process.