Fun with permissions! 404’s can be caused by lack of server read permissions on specific files and/or folders. So it’s worth checking your permissions out. Here’s a guide:
a) Set permissions on all your WordPress files to 644. I mean *all* of the files. Every single one.
b) Set all the folders you see to 755. Again, every single one.
c) If you want to be able to use the theme editor built into WordPress, set the files for your theme to 664. You may need to make them 666, depending on your server’s configuration.
d) You may need to make other files and/or folder writable on a case by case basis, depending on plugins and what have you. If this happens, use the lowest one of these numbers that actually works: 644, 664, 666, 755, 775, 777.