int pin_led = 21;
int pin_boton = 4;

void setup() {
  pinMode(pin_led, OUTPUT);
  pinMode(pin_boton, INPUT_PULLUP);
}

void loop() {
  int estado_boton = digitalRead(pin_boton);

  if (estado_boton == LOW) {
    digitalWrite(pin_led, HIGH);
    delay(100);
    digitalWrite(pin_led, LOW);
    delay(100);
  } else {
    digitalWrite(pin_led, LOW);
  }
}