The conduction mechanisms and trapping effects at SiO2/4H-SiC interfaces in metal-oxide-semiconductor field effect transistors (MOSFETs) were studied by Fowler-Nordheim (FN) tunnelling and frequency dependent conductance measurements. In particular, the analysis of both MOS capacitors and MOSFETs fabricated on the same wafer revealed an anomalous FN behavior on p-type implanted SiC/SiO2 interfaces. The observed FN instability upon subsequent voltage sweeps was correlated to the charge-discharge of hole trap states close the valence band edge of 4H-SiC. The charge-discharge of these traps also explained the recoverable threshold voltage instability observed in lateral MOSFETs.