void setup() {
  // set the pin modes
  pinMode(A4, OUTPUT);
  pinMode(A2, INPUT);
}

void loop() {
  // if button being pressed, blink the LED
  if (!digitalRead(A2))
  {
    analogWrite(A4, 256);
    delay(100);
    analogWrite(A4, 0);
    delay(100);
  }
  // if button not pressed, fade the LED
  else
  {
    for (int i = 0; i < 256; i++) {
      analogWrite(A4, i);
      delay(1);
    }
    for (int i = 256; i > 0; i--) {
      analogWrite(A4, i);
      delay(1);
    }
  }
}