try 
       { 
         StreamReader sr = new StreamReader("test.txt"); 
         StringBuilder sb = new StringBuilder(); 
         while (!sr.EndOfStream) 
         { 
           string str = sr.ReadLine(); 
           string[] temp = str.Split(' '); 
           for (int i = 0; i  < temp.Length; i++) 
           { 
             if (temp[i].Length != 4) 
               temp[i] = temp[i].PadLeft(4, ' '); 
           } 
           str = string.Join(" ", temp); 
           sb.Append(str + "\n"); 
         } 
         sr.Close(); 
         string[] result = sb.ToString().Split("\n".ToCharArray()); 
         StreamWriter sw = new StreamWriter("test.txt", false); 
         foreach (string s in result) 
           sw.WriteLine(s); 
         sw.Close(); 
       } 
       catch (Exception e) 
       { 
         Console.WriteLine(e.Message); 
       } 
 

StreamReader sr = new StreamReader(@"D:\1.txt");

using (StreamWriter sw = new StreamWriter(@"D:\2.txt", true))
{
    while (sr.Peek() != -1)
    {
        string[] temp = sr.ReadLine().Split(' ');
        for (int i = 0; i < temp.Length; i++)
            temp[i] = temp[i].PadLeft(4, ' ');
        string s = string.Join(" ", temp);
        sw.WriteLine(s);
    }
}
sr.Close();