// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0;        // will store last time LED was updated

// constants won't change:
const long interval = 500;           // interval at which to blink (milliseconds)

int pushButton1 = 2;
int pushButton2 = 4;
int pushButton3 = 6;
int pushButton4 = 11;

int buttons[4] = {pushButton1, pushButton2, pushButton3, pushButton4};


void setup() {
  // set the digital pin as output:
  Serial.begin(9600);
  pinMode(pushButton1, OUTPUT);
  digitalWrite(pushButton1, LOW);
  pinMode(pushButton2, INPUT_PULLUP);
  pinMode(pushButton3, INPUT_PULLUP);
  pinMode(pushButton4, INPUT_PULLUP);
}

void loop() {  
  bool received_input = false;
  for (int ii = 0; ii < 4; ii++)
  {
    for (int jj = 0; jj < 4; jj++)
    {
      if (jj == ii)
      {
        pinMode(buttons[jj], OUTPUT);
        digitalWrite(buttons[jj], LOW);  
      }
      else
      {
        pinMode(buttons[jj], INPUT_PULLUP);
      }
    }
    for (int jj = ii; jj < 4; jj++)
    {
      if (!(ii == jj) && !digitalRead(buttons[jj]))
      {
        for (int kk = 3; kk >= 0; kk--)
        {
          if (kk == ii || kk == jj)
          {
            Serial.print(1);
          }
          else
          {
            Serial.print(0);
          }
        }
        Serial.print("\n");
        received_input = true;
        break;
      }
    }
    if (received_input)
    {
      break;
    }
  }
}