using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var s = "123456789012";

        var list = Enumerable

            .Range(0, s.Length/4)

            .Select(i => s.Substring(i*4, 4))

            .ToList();

        var res = string.Join(" ", list);

        Console.WriteLine(res);

    }

}