If you get a 403 error when using proxypass on debian make sure you enable (a2enmod) the ‘proxy_http’ module as well as the ‘proxy’ module.
To send all request to a single mongrel instance use the following syntax:
<VirtualHost *>
DocumentRoot /var/www/yourwebsite
ProxyPass / http://localhost:4000/
ProxyPassReverse / http://localhost:4000/
<VirtualHost>