I encountered a similar issue. I believe the root of the issue may be that your parent portal's website is not locked down enough i.e., from within IIS on main parent portal's site it's still allowing anonymous connections.  Anything under the core framework/directory is accessible i.e., portal4.   So even though your other site may be secure, those files still seem to be visible (and found by google) and those files are passed to asp.net and served up by IIS.  With this access open google's bots are able to search your site and add to their search directory and results.

So check your parent's site security settings in IIS and disallow anonymous (which should stop/minimize google finding your files.  Also as others have mentioned store your files in a "secure folder" within DNN.  This tacks on .resource to all your files in that directory treating them as resource files which won't be served up by IIS.

Also, ensure to have your google account setup, so you can sign into google webmaster tools and use the "removeurl" tool to remove the offending urls from google's directory.  You may need to do a few different searchs to ensure you find all the possible searches that are now visible on the internet and then remove them from google's "search results" and "directory".  Note: that it will take about 1-2 days for google to clear these links from their directory and search results.

Also google has a robots.txt that you can save to your webserver to tell google not to search your site.  So if you're very security conscious this may be an option for you.

Hope this helps