Write a C# Sharp program to create a new list from a given list of strings where each element is replaced by 4 copies of the string concatenated together

using System;
using System.Collections.Generic;
using System.Linq;

namespace exercises
{
   class Program
    {       
        static void Main(string[] args)
        {               
           List<string> mylist = test(new List<string>(new string[] { "1", "2", "3" , "4" }));                  
           foreach(var i in mylist)
            {
               Console.Write(i.ToString()+" ");
            }     
        }                  
        public static List<string> test(List<string> nums_str)
          {
            IEnumerable<string> s = nums_str.Select(x => x = x + x + x + x);
            return s.ToList();
          }    
    } 
}