Use different 404 page on front-end and back-end

If your admin post does not exist, redirect as you did.

1. create Admin404.php file in /application/controllers/admin/ folder.
   
2. make class Admin404 in Admin404.php file
   
3. create index() function in Admin404 class
   
4. call your custom admin 404 error view in index() function with
   
   PHP Code:

   $this->load->view('admin/error_404'); 

5. create error_404.php view file in /application/views/admin/ and put your custom stuff in it.
   

That is it. It should work like you want.