Customize magento 404 page, change 404 page layout, remove compare product ads on 404 page, call product on 404 page, add search form on 404 page.

Go to admin >> CMS >> Pages
Edit “404 Not Found 1” row.

// Inside content tab
you are able to add edit remove 404 page Data.

// Inside Design tab
You can add block (search, call product,contact us, etc) in design tab >> inside “Layout Update XML”.
Change layout of 404 page inside Design tab.

If you want to remoe compare product ads on 404 page. paste below code on “Layout Update XML”

<reference name="right">  
<remove name="catalog.compare.sidebar" /> //remove compare product block
<remove name="wishlist_sidebar" /> //remove wishlist block
</reference>