Someone

<IfModule mod_rewrite.c>
  <IfModule mod_ssl.c>
    <Location webmail4.mydomain.com>
      RewriteEngine on
      RewriteCond %{HTTPS} !^on$ [NC]
      RewriteRule . https://%{HTTP_HOST}%{REQUEST_URI}  [L]
    </Location>
  </IfModule>
</IfModule>