Comparative studies of gate oxides on a N+ pre-implanted area (Ninterface~ 1x1019cm-3) and on a virgin Si face 4H-SiC material (Ninterface~ 1x1016cm-3) have been undertaken by means of Capacitance-Voltage (CV) characteristics, performed at different temperatures and frequencies, and Thermal Dielectric Relaxation Current technique. In the non implanted samples, the stretch out of the CV curves get larger as the temperature is lowered to 150K, while for lower temperatures the CV characteristics become steeper and some discontinuities occur. These discontinuities are specific for the non-implanted sample and are associated with charging of the fast near interface states (NIToxfast) via a tunneling from the shallow interface states (Dit). The tunneling from the shallow Dit to NIToxfast supress the ac response of Dit, which is recovered only after most of the NIToxfast are charged with electrons.