This usually occurs when you're tying to navigate to a page on your site that doesn't exist, or in this case, mis-named.
When you initially set up your Magento site, you go through the installation steps (i.e. setting your currency and location, setting up your database, etc.). On the same page where you specify your database name, there's a section near the bottom of the page called "Web Access options". It resembles the following:
As the caption specifies, any changes you make here during the setup, is stored in the app/etc/local.xml file. That includes your database information and most importantly, your admin path.
The admin path is the URL to the backend of your Magento site.
If you encounter a 404 when trying to access the backend of your site, make sure you're using the correct admin path. You can check to see what your admin path is set to by going to app/etc/local.xml and locating the information for adminhtml (as shown above).
Please wait... it will take a second!