Why did my LED resistor burn while lighting four LEDs in series?
6.8 volts seems awfully high for a single LED. Are you sure that 6.8 is not the number for all four LEDs? That would make it 1.7 volts per LED, which is more reasonable for a red LED. And that would mean that you are currently pushing 172 milliamps, or almost 3 watts through your resistor.

If that is the case, you should lower your power supply to less than 20 volts (maybe 12 volts) to keep from destroying the gate of your MosFET (M2).