No, you should expose "127.0.0.1:4000:80", then use the config snippet I’ve linked to and proxy into the container.
Let me repeat: you can not run Discourse on port 80 while another webserver is running on your host system. It’s simply not possible.
As for Apache’s default page appearing, did you wrap that block in a <VirtualHost> block? For Apache 2.2: did you enable name-based virtual hosting?
Also, and I cannot stress this enough, if you’re going to be a server admin, you should understand what you’re doing or you’re going to put yourself and your users at risk. Be responsible and RTFM. (Read The F…riendly Manual)