**Interesting task indeed! Here is my solution - the explanation is in the comments:**

using System;
class Program
{
static void Main()
{
int n = int.Parse(Console.ReadLine());//the user inputs 4 digits number
int firstNumber = (n / 1000) % 10;//divide by 1000 >> then module dividing by 10 to get the 1st number of the 4th digit number
int secondNumber = (n / 100) % 10;//divide by 100 >> then module dividing by 10 to get the 2nd number of the 4th digit number
int thirdNumber = (n / 10) % 10;//divide by 10 >> then module dividing by 10 to get the 3rd number of the 4th digit number
int fourthNumber = (n % 10);//Module dividing by 10 to get the 4th number of the 4th digit number
//Console.WriteLine("{0},{1},{2},{3}",firstNumber, secondNumber, thirdNumber, fourthNumber);//Check whether it works!
Console.WriteLine("The sum of the digits is: {0}", firstNumber + secondNumber + thirdNumber + fourthNumber);
Console.WriteLine("Reversed order: {0}{1}{2}{3}", fourthNumber, thirdNumber, secondNumber, firstNumber);
Console.WriteLine("Last digit upfront: {0}{1}{2}{3}", fourthNumber, firstNumber, secondNumber, thirdNumber);
Console.WriteLine("Exchanges the second and the third digits: {0}{1}{2}{3}", firstNumber, thirdNumber, secondNumber, fourthNumber);
}
}