When you have a RHN license, it is valid for all versions. You can even have multiple versions on a single machine (using virtualization) and they are ALL covered. If your version of out of support, you may find that you cannot get new software patches or packages. Security soon becomes a distressing issue, but there are no LEGAL issues and you can keep running your version as long as you wish.
RHEL 4 is near EOL and RH is sending notices to major clients, but you are far from EOL for RHEL 5.
NOTE: if you really want to justify, record the information on release lifespan available from the RH software web site. That is the direct and official statement.