Use themes for 404 page in .htacess

In the current template that I am using I have customized an error 404 page. I would like to make it so that this is the defult 404 page by using .htacess. I was wondering if I could use the WP_USE_THEMES comand or anything like it so that the 404 template always matches the rest of my blog. Is this possibe? If so, how is it done.

Many thanks,
Cron