const int buttonPin = 4;
const int ledPin = 2;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLDOWN);
}

void loop() {
  int buttonState = digitalRead(buttonPin);

  if (buttonState == HIGH) 
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
}