Websites pointed at your IIS webserver that were not created in IIS will return a 404 error.
Instead of showing a 404 error, you could create a catch-all website with new contents. The contents of the catch-all website will be returned whenever a domain is pointed to your server, but does not match with a website's host name.
Follow the steps below to create an IIS catch-all site.
The contents of this folder will be shown as the catch-all website. You can create C:\inetpub\default, for example. Put any website files (HTML, etc) in this folder.
To create the website in IIS, go to "Sites" and click "Add Website...". Enter all the fields like you normally would, but leave "Host name" empty. As for the "Site name", call it "Default" or whatever you want. Since the "Host name" is empty, IIS will consider this the catch-all site.
Congratulations, you now have a website that will be shown instead of a 404 error!