Your site’s permalink (WordPress Admin > Settings > Permalink) is set as Default, please change it to any of the other option available below. e.g: Day and name, Post name etc.
Then you’ll be able to access the board instead of the 404 error.
Another way is troubleshooting:
WordPress sites can complicate the 404 troubleshooting process. Why? WordPress is a content management system that processes its own internal rewrite array as a part of its permalinks feature.
The first step to troubleshooting 404s is to figure out whether the 404 is being caused by the web server or by WordPress.
Static file
If your file is static (e.g. a jpg image) open your FTP client and verify that the file exists.
As an example, let’s say that the URL http://example.com/wp-content/uploads/picture.jpg is producing a 404 error. You will want to:
Open FTP Client and connect to your server (if you don’t know how to do so, read this.)
Navigate to the file’s location
Verify that the file exists
If it is does not exist, you have found the source of your 404.
However, it it does exist, yet pulling the URL up in a browser results in a 404 error, continue on to find out whether the issue is web server-based or WordPress-based.
Reference Link: http://websynthesis.com/fixing-wordpress-404-errors/