const unsigned int BUTTON_PIN = 4;
const unsigned int LED_PIN    = 13;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}
int old_button_state = LOW;
int led_state = LOW;

void loop() {
  const int CURRENT_BUTTON_STATE = digitalRead(BUTTON_PIN);
  if (CURRENT_BUTTON_STATE != old_button_state &&
      CURRENT_BUTTON_STATE == LOW)
  {
    if (led_state == LOW)
      led_state = HIGH;
    else
      led_state = LOW;
    digitalWrite(LED_PIN, led_state);
    delay(50);
  }
  old_button_state = CURRENT_BUTTON_STATE;
}