Hi,
I did not make full tests of the latest URE version against older WordPress versions.
But quick look shows that version 4.21 works in general with WP 3.9.9. In order to remove required WP version warning, open the file user-role-editor-pro.php, go to the line #48 and replace 4.0 there to 3.9.9, in order to get this:
$ure_required_wp_version = '3.9.9';
I raised required WP version to 4.0 starting from URE version 4.19. If you need it, I may send it to your email.
