You can configure YUM for RHEL3 and RHEL4 machines to use the YUM repository configured for CENTOS. The directory structure is same for RHEL and CENTOS. To know more about CENTOS follow the link
NOTE: Be carefull dont go for "full update" as this will change the G-NOME panel for RHEL machine to CENTOS and your system will then appear like CENTOS machine. I will suggest to update only rpm packages you needed not as group.
YUM SRC package for RHEL4 will be in the link
To configure the YUM repository for RHEL3 and RHEL4
If you have problem with proxy server for resolving with Domain controller use the following environmental variable in bah shell
Note: Dont make the above entry in any profile as your domain password will expire after few days and it leads to Domain password lock.
Hope this is will give you idea to proceed.