First-principles calculations are used to investigate the Shockley partial dislocations in 4H-SiC. We show that both dislocations can sustain the asymmetric and symmetric reconstructions along the dislocation line. The latter reconstructions are always electrically active. In particular, the Si(g) 30 degrees partials can explain the optical activation energy for the dislocation glide at -2.4 eV above the VB, the narrow peak at 2.87 eV and the broadband at -1.8 eV found in photoluminescence spectra. Further, we propose a new model to explain the stability of the symmetric reconstructions and the enhancement of the dislocation velocity in SiC.