Problem with ERROR (404.ph) site

Hello people!
In urgent need of your help – I am super desperate. Since yesterday I am trying to get my 404 page to work. I have the 404.php in my template folder. However, if I make a mistake, is just a blank page – not the custom 404 page.

I’ve really tried everything – htaccess. 100x changed until nothing worked. I’m on it but just not. I’ve finally now asked my provider for the problem – but could not find a problem.

Why is that?
Do I activate a 404.php in wordpress only or something?

Sorry, I am beginner and my english isnt very good. But there are no german forums, that could help me. I do come with WordPress really well, but this problem is really currently too high for me - do not understand why this does not work. I cant call the 404.php even if I enter the exact path.

Please, please, please help!

my .htaccess

# BEGIN WordPress

# END WordPress

the 404.php

<?php get_header(); ?>
<div id="content">
     <div class="cont-box bot-indent">
        <div class="top"></div>
       <div class="border-left">
            <div class="border-right">
                 <div class="tail">
                    <div class="inside">
                       <div id="error404" class="clearfix">
                          <div class="error404-num grid_7">404</div>
                   <div class="grid_4">
                      <hgroup>
                         <h1>Sorry!</h1>
                      <h2>Page Not Found</h2>
                    </hgroup>
                      <h4>The page you are looking for might have been removed, had its name changed, or is temporarily unavailable.</h4>
                    <p>Please try using our search box below to look for information on the internet. </p>
                    <?php get_search_form(); /* outputs the default WordPress search form */ ?>
                      </div>
                   </div><!--#error404 .post-->
                     </div>
           </div>
           </div>
       </div>
       <div class="bot"></div>
           </div>
         <div class="container_12 clearfix" id="content-bot">
      <?php if ( ! dynamic_sidebar( 'Content Bottom' ) ) : ?>
      <!--Widgetized 'Request' for the home page-->
      <?php endif ?>
   </div>
</div><!--#content-->
<?php get_footer(); ?>

The .htaccess file is located at:
http://www.example-domain.com/wp/

The 404.php file is located at:
http://www.example-domain.com/wp/wp-content/themes/themexyz