Just tried to install 4.0RC2 and I ran into the same exact problem.

I changed line 150 of install.php to:

        $result = mysql_query("INSERT INTO bigtree_users (`email`,`password`,`name`,`level`) VALUES ('$cms_user','$enc_pass','Developer','2')");
        if (!$result) {
            die('Invalid query: ' . mysql_error());
        }

And the error I received was:

Invalid query: Field 'permissions' doesn't have a default value

I updated line 150 of install.php to:

mysql_query("INSERT INTO bigtree_users(`email`,`password`,`name`,`level`,`permissions`,`alerts`,`daily_digest`,`change_password_hash`) VALUES ('$cms_user','$enc_pass','Developer','2','','','','')");

And the install created the CMS user as expected.