A novel technique is applied to extract the band-to-band absorption coefficient alpha(bb) in 4H-SiC. The fine structure of the absorption spectra alpha(bb)(hnu) in the vicinity of E(g) is revealed for the first time in the temperature range 70 - 455 K. Our results indicate that indirect optical transitions are assisted by dominant LA phonons with energy of similar to75 meV. The thermal band-gap narrowing effect is estimated in the order of 100 meV at 455 K.