Hi.
When i got hold of my rhel4 ES, the company i work for had to pay a
one year subscription fee that entitled me one year's support from RedHat for one computer.
I'm not so sure that RedHat will let you update with-out having
a subscription.
The best way would be to go to RedHat's site and check it out.
http://www.redhat.com
Hope this helps.