int mosfetpin = 4;

// the setup function runs once when you press reset or power the board
void setup() {
 // initialize digital pin LED_BUILTIN as an output.
 pinMode(mosfetpin, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
 digitalWrite(mosfetpin, HIGH);   // turn the LED on (HIGH is the voltage level)
 delay(3000);                       // wait for a second
 digitalWrite(mosfetpin, LOW);    // turn the LED off by making the voltage LOW
 delay(3000);                       // wait for a second
}