int pins[] {7,6,5,4};
const int NUMBER_OF_PINS = sizeof(pins)/sizeof(pins[0]);
const int NUMBER_OF_LEDS = NUMBER_OF_PINS * (NUMBER_OF_PINS-1);
byte pairs[NUMBER_OF_LEDS/2][2] = {{0,1}, {1,2}, {0,2}, {2,3}, {1,3}, {0,3} };

int letter[] = {0,1,2,3,6,8,9,10,11};

void setup() { 
  
}

void loop() {
  for(int i = 0; i < 9; i++) lightLed(letter[i]);
  delay(10);
}




void lightLed(int led){
  int indexA = pairs[led/2][0];
  int indexB = pairs[led/2][1];
  int pinA = pins[indexA];
  int pinB = pins[indexB];

  for (int i = 0; i < NUMBER_OF_PINS;i++){
    if (pins[i] != pinA && pins[i] != pinB){
      pinMode(pins[i], INPUT);
      digitalWrite(pins[i], LOW);
    }
  pinMode(pinA, OUTPUT);
  pinMode(pinB, OUTPUT);
  }
  if (led % 2 == 0){
    digitalWrite(pinA, LOW);
    digitalWrite(pinB, HIGH);
  }
  else
  {
    digitalWrite(pinA, HIGH);
    digitalWrite(pinB, LOW);
  }
}