Posts and Pages: 404 Page not found

Did you try to re-save Permalinks?

Navigate to: WordPress Dashboard > Settings > Permalinks: Click “Save Changes” and then click on the link of the page that should work from the “Edit Page” of that page.

Also review Permalinks settings if they are correct.

If it still doesn’t work, you can try to debug the generated permalinks with Debug This plugin.

After activating this plugin go to: Admin Bar > Debug This > Query > Rewrites and check if the rewrite rules are correct and exist for the url that should work.