Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at the hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.