Do you notice this;?release=$releasever&arch ???? Check your redhat-release file, does it say something like;
$ cat /etc/*release
CentOS release 4.3 (Final)
If no, then how do you expect to get the repo info (think about it)?????
Yes CentOS is binary compatible with Red Hat Enterprise Linux 4, but this does not mean they are exactly the same. Yum can be used to get some packages like those from rpmforge along with other places but not from Red Hat;
http://dag.wieers.com/packages/rpmforge-release/
For up2date either pay for the Red Hat subscription or switch to CentOS.