const int GPIO_Pins[] = {5, 4, 3, 2};   // pin nos 2 to 5 represent four bit binary number
// bit bit 0 = pin no 5, bit 1 = 4, b2 = 3 and bit 3 = 2
// You can use same colour LEDs for four bits 

const int numLEDs = 4;

const int delayLEDsNum = 2000; // in milliseconds
const int delayLEDsEnd = 3000; // additional delay of 3 seconds while highest number is being displayed

void setup() {

  byte temp;
    
  for(int led = 0; led < numLEDs; led++)
  {
    pinMode(GPIO_Pins[led], OUTPUT);   // Set the LEDs in OUTPUT mode
    digitalWrite(GPIO_Pins[led], LOW); // clear all the bits initially
  }
  
}

void loop() {
  // put your main code here, to run repeatedly:
  static byte num = 0; 
  displayNum(num);
  displayNum(~num);
  num++;  

  if(num > 15){
    num = 0;
    delay(delayLEDsEnd);
  }
    
} // end of loop()

void displayNum(int num){
  byte mask = 0x01;  // only the LSB is one, all other 7 bits are zeros
  byte tempMask;
  
   for(int i = 0; i < numLEDs; i++){
    tempMask = mask << i;
    
    if( (num & tempMask) != 0){
      digitalWrite(GPIO_Pins[i], HIGH);
    }
    else{// if the bit is zero
      digitalWrite(GPIO_Pins[i], LOW);      
    }
   } // end of for loop
   
     delay(delayLEDsNum); 
     
} // end of displayNum()