I think the simple answer is that you can’t. There are a number of things wrong with this circuit, starting with the connection of power to the 5V pin. The input power (greater than about 7 volts) should be going to VIN (assuming you don’t have the USB connected in which case the 5V is coming from the USB connection), the 5V pin is the output of a voltage regulator. You might be able to make a circuit where a PNP transistor connected across the power switch (assuming the power switch is a push button) was held on by writing a 0 to the D4 port. You would need to hold the switch down til the arduino boots and sets the port to 0 (and would need a resistor between the base of the transistor and the I/O pin to control the base current), but that won’t work if the USB is connected.
Peter