const unsigned char buttonPin = 7;

void setup() {
  Serial.begin(115200);
  Serial.println("Hello\n");

  pinMode(buttonPin, INPUT_PULLUP);
}

# define DEBOUNCE 50    // milliseconds to ignore switch
unsigned char buttonState, prevButtonState;
unsigned long lastButtonTime;

unsigned char state;
unsigned char printed;

void loop() {

  unsigned long now = millis();

  if (now - lastButtonTime > DEBOUNCE) {     // switch still hot from last time? ignore it.

    buttonState = digitalRead(buttonPin);
    if (buttonState != prevButtonState) {
      // button has been pressed or released
      if (buttonState == LOW) {
        // move to the next state
        state++;
        if (state == 4) state = 0;    // reset
        printed = false;
      }
         // remember the button state and time
      prevButtonState = buttonState;
      lastButtonTime = now; 
    }
  }
  
  if (!printed) {
    Serial.println(state);
    printed = true;
  }
}