404 not found!

No. the default controller is *only* executed if your original URL maps to a folder without an explicit controller identified.

localhost/subit_backend.php/register will look for a subit_backend.php file in your document root.
localhost/subit_backend/register will look for a Subint_backend controller inside application/controllers, and will try to invoke its register method.

It sounds like you would benefit from carefully re-reading the CI user guide sections on routing.