FileRangeRenamer.cs

using System.IO;
using System.Linq;

class FileRangeRenamer
{
    static void Main(string[] args)
    {
        var folder = @"D:\Photos\June12\Clouds1";
        var files = Directory.GetFiles(folder, "*.CR2").OrderBy(f => f).ToArray();
        foreach (var file in files)
        {
            var fileName = Path.GetFileNameWithoutExtension(file).Substring(4);
            if (fileName.StartsWith("9"))
            {
                var newName = "IMG_" + (int.Parse(fileName) - 1000).ToString();
                File.Move(file, Path.Combine(folder, newName + ".CR2"));
            }
            else if (fileName.StartsWith("0"))
            {
                var newName = "IMG_" + (int.Parse(fileName) + 9000).ToString();
                File.Move(file, Path.Combine(folder, newName + ".CR2"));
            }
        }
    }
}