So make an html page and copy all of your code expect the content section. Put
Code:
<h2>404 Error page</h2>
<p><!--#echo var="REQUEST_URI" --> does not exist. <a href="#" class="und"><strong></strong></a>Please check that address and try again. </p>.
Boom you have a 404 page that looks exactly as your home page.
I am sorry if I am not understanding what you are asking. As for the 404 page the redirects to another page, I am unsure as to how to accomplish that.
Use caution with these error pages, because if not done properly you can get penalized by the search engine for having duplicate content.