using System;

public class Program

{

    public static void Main()

    {

        string cardNumber = "4711111111111111";

        Console.WriteLine( GetCardNumber(cardNumber) );

    }

    public static string GetCardNumber(string cardNumber)

    {

        if(string.IsNullOrWhiteSpace(cardNumber))

        {

            return string.Empty;

        }

        string lastFour = cardNumber.Substring(cardNumber.Length-4,4);

        return lastFour.PadLeft(cardNumber.Length, 'X');

    }

}