Except, we don't maintain the 4.1, 4.2, 4.3, 4.4 trees indefinitely, we maintain a 4 tree ... that is always updated to be just like running up2date upstream.
You are confusing moving from CentOS-2 to CentOS-3 to CentOS-4 with moving from CentOS-4.1 to CentOS-4.2 to CentOS-4.3. They are not the same.
Until RH releases 4.5.0,4.5.1,etc with plans to update in similar fashion to what scientific linux has done. That should quite thoroughly confuse the hell out of everyone.