Can't write lock bits on ATMega328p-pu
To use the Arduino as ISP you should specify avrisp as the programmer:

avrdude -c avrisp -p m328p -P COM4 -b 19200 -U lock:w:0xFC:m


See eg the Sparkfun tutorial on Arduino as ISP. 

I can't confirm it right now but I believe the arduino programmer uses the serial protocol of the Arduino bootloader, though I thought that was essentially STK500, so I don't know why that is needed as a separate option. Maybe it is just a subset of STK500 that is implemented in the bootloader.