If you block port 81 outbound, centos-yumconf-4-4.4.noarch.rpm will cause your yum to stop working for CentOS-4.
We were serving mirrorlists on port 81 for our new update system, but since this caused some users problems, we are now serving mirrorlists on both port 80 and 81.
centos-yumconf-4-4.5.noarch.rpm has released to change the default configuration to port 80 for mirrorlist retrieval.
If your yum is not working you can get centos-yumconf-4-4.5.noarch.rpm here:
http://mirror.centos.org/centos/4/updates/i386/RPMS/centos-yumconf-4-4.5.noa...
This is a noarch RPM and should work on any centos-4 version.
You can install it after download with this command:
rpm -Uvh centos-yumconf-4-4.5.noarch.rpm