In my custom 404 page do I need to include the code <?php header ("HTTP/1.1 404 Not Found"); ?> I do have my ErrorDocument setup in htaccess
Since you have specified it in .htaccess as 404 document then I think you do not need additional 404 status message. Imagine if you had html file as 404 document, then you couldn't ever specify custom header, since html page returns only html code. You can always put this to test after configuring and check the status code the server returns in your case. You can do that with various software/tools or even through some online tools on various websites.