How to fix 404 error appearing after changing permalinks settings to pretty permalinks from WordPress dashboard > settings > permalinks.
The most common situation is that that after changing permalinks to pretty permalinks admin area is accessible as well as the front page, but all posts and other pages gives 404-error.
The most common solution for the error is just re-save permalinks and try again, it should flush permalink rewrite rules.
If 404 error still appears, try to re-upload / update .htaccess file manually with the contents below. It should fix the problem.
You can upload & edit .htaccess file using your favorite ftp-client or handy build-in File Manager provided with control panel, if you are using for example Plesk or CWP7.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress