.htaccess is not returning HTTP 404 Error Page

Please, I’m tired of trying to find and fix the error inside the .htaccess which is returning a page of the blog instead the Error HTTP 404 Page. I have been penalized already in Google for this. I need somebody to help me telling me what to do.

This is what I have in my .htaccess code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

My blog is in the root directory
Please, somebody, send me the right code.