bool flag = false;
bool led_state = false;

void setup() {
  Serial.begin(115200);
  pinMode(4, OUTPUT);
  pinMode(13, INPUT);
}

void loop() {
  if(digitalRead(13) == 1 && flag == false){
    flag = true;
    led_state = !led_state;
}

if(digitalRead(13) == 0 && flag == true){
  flag = false;
}

digitalWrite(4, led_state);
delay(50);
}