How to hide wordpress error from a non logged guest rather to show 404 error while trying wp-admin?

Solution:

You can use the Lockdown WP plugin to hide the /wp-admin section and return a 404 error for users that are not logged in. It will also rewrite the login as you have already done.

Keep in mind that there are still many give-aways that you are using WordPress, such as the wp-content folder and other telltale files, if people actually want to find out what you are running.