I just extended a Web Application and provided it a new URL and host name. After completion (and altering my host file to direct the new url) I can see the new web sites created in IIS on each of the servers but cannot navigate to it.
This error (HTTP 404 Not Found) means that Internet Explorer was able to connect to the website, but the page you wanted was not found. It's possible that the webpage is temporarily unavailable. Alternatively, the website might have changed or removed the webpage.
Can you provide an anonymized example of your URLS? For example: http://portal.mysite.com, http://mysite.com, etc.
My first thought is you didn't create a site collection when you created the web application. Since you are extending an EXISTING web app, this may not be the case.
Second, it may be a DNS issue. I have run into issues with DNS and Alternate Access Mapping. I could always resolve the site from the web server, but not from any other machine, because the AAM URL was not recorded in DNS.
If you provide more details, we can probably narrow down the problem.
If it was straight away after creating it, then SharePoint & IIS often require an iisreset before everything is happy and the pages will work.
I had this issue today too. I haven't yet tried to solve it, but while searching for a solution, landed on your question. You might have already solved this, but there is no accepted answer. So, I thought I'll post my findings. I found this link and it seems like a good explanation of the issue. I did create webapps before too and they worked fine. I went through their differences and couldn't find any apparent issues. After finding this link, it sounds like the name could be the issue. Then I found this link and it could help with the solution.
Check the binidng in IIS and Alternate access mapping... that is necessary
