Normally-off 4H-SiC MOSFETs are used to build NMOS logic gates intended for high temperature operation. The logic gates are characterized between 25°C and 500°C. Stable gate operation for more than 200h at 400°C in air is demonstrated. The excellent MOS reliability is quantified using I-V curves to dielectric breakdown and constant voltage stress to breakdown at 400°C. Although the effective tunneling barrier height B for electrons lowers to 2eV at 400°C, the extrapolated lifetime from constant voltage stress to breakdown measurements is longer than 105h at 400°C for typical logic gate operating field strength of 2MV/cm.