As far as I know you can't use the Webfarm-Framework for balancing between different ports on one Server. Maybe because from a failure safety perspective it dosn't make sence. A workaround is to add some additional IPs to your Webserver and configure your 4 web sites bindings to listen on the same port but on different IPs. So you cann set up a web farm with 4 different IPs as servers which in fact are locatet on the same physical machine.
hope this helps. Best regards, Peter