#define BUTTON_PIN 4
#define BUZZER_PIN 18

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP); // internal pull-up
  pinMode(BUZZER_PIN, OUTPUT);
}

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

  if (buttonState == LOW) {   // button pressed
    digitalWrite(BUZZER_PIN, HIGH);
  } else {                    // button released
    digitalWrite(BUZZER_PIN, LOW);
  }
}