CentOS 4.2 offers yum to do updates of the operating system.
Prior to using yum, you must import the RPM-GPG-KEY for CentOS-4.2 like this
rpm --import /usr/share/doc/centos-release-4/RPM-GPG-KEY
For more Information, see these Release-Notes, provided {and copyrighted} by Red Hat®,Inc. The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-4. CentOS-4 is built from publicly available, open source SRPMs.