using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

​

public class Program

{

    private class PaymentInstrument

    {       

        public string AccountNumberLastFourDigits { get => (AccountNumber.Length < 4) ? AccountNumber : AccountNumber[^4..]; }

        public string AccountNumber { get; set; }

    }

    private class CheckInstrument : PaymentInstrument

    {

        public override string ToString()

        {

            return string.Format(

                "AccountNumber = {0}, AccountNumberLastFourDigits = {1}",

                AccountNumber, AccountNumberLastFourDigits

            );

        }

    }

    public static void Main()

    {

        var check = new CheckInstrument

        {

            AccountNumber = "987654321"

        };

        Console.Write(check.ToString());

    }

}