it would be best to talk to your Red Hat license vendor to ensure you are getting accurate information. In short, I read it as you have mentioned. That under a base license, you are allowed to have up to 4 guests licensed with rhn and thus getting updates.
Don't be confused with what is supported vs. what is possible. It is "possible" to do a lot of things. For example I could run 10000 client machines (physical/virtual) and only have 1 license and use that license to create my own repository and have everyone else update from there. Trouble is it is not supported, I won't get Red Hat support for each machine if/when needed, not to mention the legalities or doing that. Companies generally want to be under support. If you are looking for ways around buying support licenses, use Centos.