There are several reasons that may couse 404 error, here there are some solutions:
1. Make sure that .htaccess file exists
Make sure that .htaccess file exists in the Joomla! site root. If .htaccess file does not exists, you may create it by renaming the htaccess.txt file included in Joomla! root directory. Also, make sure that the file size is higher than 2.5 KB, if not, get a clean htaccess.txt file from a Joomla installation package then rename it on your FTP.
2. Check the rewrite rules .htaccess file
Make sure that the RewriteCond and RewriteRule directives are uncommented in the section for 3rd party SEF modules (almost end of the .htaccess file) - this only applies to Joomla 1.0.x. In Joomla 1.5.x there is only one set of rules that is already uncommented by default.
3. Check RewriteBase value and adjust it if needed
Finally, make sure to set the RewriteBase value to match the root URL of your Joomla! installation at the server.
This means the path to root without the server name - e.g. if your Joomla! is installed as http://www.domain.org/somedir/myjoomlaroot, that the right setting would be RewriteBase /somedir/myjoomlaroot). If your site is located in domain root, the RewriteBase may either stay commented our or have / value. However, even if in root some servers require uncommenting it and setting explicit value (then set it simply to RewriteRule /).
4. Moving from other SEF components
In such a case, most of the URLs that comes from outside of your site (Google or other sites) returns 404 error because the URL structure has been changed. I mean AceSEF generates different URLs than your old SEF component so they are reported as not found (404) from AceSEF.
To fix this, we suggest you to export your URLs from the previous SEF component
then import them to AceSEF so that all your old URLs could work fine.
5. Use "Moved URLs" section
Use the "Moved URLs" section if non of the above solutions worked for you. Go to "404 URLs", copy the not found URL, go to Moved URLs, click on "New" then paste the copied URL to "From URL" field and enter the target/working SEF URL in "To URL" field. For more details:http://www.joomace.net/support/docs/acesef/user-manual/urls#moved