Change internally 403 Error to 404 Error

I have the htaccess file within the folder in the domain I want to deny access to all users, execept one IP, after that
redirects Error 403 resulting to a 404 error custom html page as shown below.

# Restringting access!
deny from all
allow from 204.127.111.105     # Example of allowed IP
ErrorDocument 403 /404.shtml  # Redirecting error 403 to error 404 page

I would like to change 403 Error and redirect to an internal 404 Error, Or, avoid the generation of 403 Error code and redirects
only to the 404Error.html custom page without any user could know that has been redirected from an original 403 Error.

I`ve tried with the code below but it shows a 403 error and the RewriteRule looks like doesn�t have any effect in the code.

# Restringting access!
deny from all
allow from 204.127.111.105     # Example of allowed IP
RewriteCond %{REMOTE_ADDR} ^205\.127\.127\.252  #Testing for 1 IP, 
RewriteRule ^(.*)$ /test.jpg [L]

May somebody help me with this issue?

Thanks in advance for any suggestion.

What does the web server log file say about this?

---------- Post updated at 00:57 ---------- Previous update was at 00:52 ----------

Also,, you might want to have a look at this:

International Customized Server Error Messages

... and more importantly:

ErrorDocument Directive