Generate Four digit sequence number using C#

Dear Programmer i have a code for Generate Sequence Number and it is working very claver, but i am facing a error related to this code.

This Code is generate like this - 

1, 2, 3, 4, 5, 6, 7, 8 .............. and so on

But is want this code like as for four digit specific - 

0001, 0002, 0003, 0004, 0005 ......and so on.

Please help me out. and my code is - 

        private int GenerateSequenceNumber()
        {
            string file2 = (Server.MapPath("test.txt"));
            FileInfo fiRead = new FileInfo(file2);
            string input = string.Empty;
            int num = 1;
            if (fiRead.Exists)
            {
                input = File.ReadAllText(file2);
            }
            if (input != string.Empty)
            {
                num = Convert.ToInt32(input);
            }
            File.WriteAllText(file2, (num + 1).ToString().PadLeft(4));
            return num;
        }